C++: Stop generating NoOp instructions for declarations of externs

Previously, where we had a function-scoped `DeclarationEntry` for an extern variable or function, we would generate a `NoOp` instruction for it. There's nothing wrong with this by itself, although it was unnecessary. However, I've hit an extractor issue (Jira ticket already opened) that commonly causes multiple `DeclStmt`s to share a single `DeclarationEntry` child on extern declarations, so removing the `NoOp` instructions is an easy way to work around the extractor issue.

Author: dave-bartolomeo

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/internal/TranslatedDeclarationEntry.qll

@@ -47,7 +47,7 @@ abstract class TranslatedDeclarationEntry extends TranslatedElement, TTranslated
 /**
  * Represents the IR translation of a declaration within the body of a function,
  * for declarations other than local variables. Since these have no semantic
- * effect, they are translated as `NoOp`.
+ * effect, they do not generate any instructions.
  */
 class TranslatedNonVariableDeclarationEntry extends TranslatedDeclarationEntry {
   TranslatedNonVariableDeclarationEntry() {
@@ -56,25 +56,20 @@ class TranslatedNonVariableDeclarationEntry extends TranslatedDeclarationEntry {
 
   override predicate hasInstruction(Opcode opcode, InstructionTag tag,
       Type resultType, boolean isGLValue) {
-    opcode instanceof Opcode::NoOp and
-    tag = OnlyInstructionTag() and
-    resultType instanceof VoidType and
-    isGLValue = false
+    none()
   }
 
   override Instruction getFirstInstruction() {
-    result = getInstruction(OnlyInstructionTag())
+    result = getParent().getChildSuccessor(this)
   }
 
   override TranslatedElement getChild(int id) {
     none()
   }
 
   override Instruction getInstructionSuccessor(InstructionTag tag,
-    EdgeKind kind) {
-    tag = OnlyInstructionTag() and
-    result = getParent().getChildSuccessor(this) and
-    kind instanceof GotoEdge
+      EdgeKind kind) {
+    none()
   }
 
   override Instruction getChildSuccessor(TranslatedElement child) {

cpp/ql/test/library-tests/ir/ir/PrintAST.expected

@@ -7828,3 +7828,40 @@ ir.cpp:
 # 1109|             Type = [LValueReferenceType] unsigned int &
 # 1109|             ValueCategory = prvalue(load)
 # 1111|     1: [ReturnStmt] return ...
+# 1113| [TopLevelFunction] void ExternDeclarations()
+# 1113|   params: 
+# 1114|   body: [Block] { ... }
+# 1115|     0: [DeclStmt] declaration
+# 1115|       0: [VariableDeclarationEntry] declaration of g
+# 1115|           Type = [IntType] int
+# 1116|     1: [DeclStmt] declaration
+# 1116|       0: [VariableDeclarationEntry] definition of x
+# 1116|           Type = [IntType] int
+# 1117|     2: [DeclStmt] declaration
+# 1117|       0: [VariableDeclarationEntry] definition of y
+# 1117|           Type = [IntType] int
+# 1117|       1: [FunctionDeclarationEntry] declaration of f
+# 1117|           Type = [IntType] int
+# 1118|     3: [DeclStmt] declaration
+# 1118|       0: [FunctionDeclarationEntry] declaration of z
+# 1118|           Type = [IntType] int
+# 1118|       1: [FunctionDeclarationEntry] declaration of w
+# 1118|           Type = [IntType] int
+# 1118|       2: [VariableDeclarationEntry] definition of h
+# 1118|           Type = [IntType] int
+# 1119|     4: [DeclStmt] declaration
+# 1119|       0: [TypeDeclarationEntry] declaration of d
+# 1119|           Type = [LocalTypedefType] d
+# 1120|     5: [ReturnStmt] return ...
+# 1117| [TopLevelFunction] int f(float)
+# 1117|   params: 
+# 1117|     0: [Parameter] p#0
+# 1117|         Type = [FloatType] float
+# 1118| [TopLevelFunction] int z(float)
+# 1118|   params: 
+# 1118|     0: [Parameter] p#0
+# 1118|         Type = [FloatType] float
+# 1118| [TopLevelFunction] int w(float)
+# 1118|   params: 
+# 1118|     0: [Parameter] p#0
+# 1118|         Type = [FloatType] float

cpp/ql/test/library-tests/ir/ir/ir.cpp

@@ -1110,4 +1110,13 @@ static void AsmStmtWithOutputs(unsigned int& a, unsigned int& b, unsigned int& c
     );
 }
 
+void ExternDeclarations()
+{
+    extern int g;
+    int x;
+    int y, f(float);
+    int z(float), w(float), h;
+    typedef double d;
+}
+
 // semmle-extractor-options: -std=c++17

cpp/ql/test/library-tests/ir/ir/raw_ir.expected

@@ -5112,3 +5112,19 @@ ir.cpp:
 # 1104|     v0_17(void)                  = ReturnVoid             : 
 # 1104|     v0_18(void)                  = UnmodeledUse           : mu*
 # 1104|     v0_19(void)                  = ExitFunction           : 
+
+# 1113| void ExternDeclarations()
+# 1113|   Block 0
+# 1113|     v0_0(void)       = EnterFunction       : 
+# 1113|     mu0_1(unknown)   = AliasedDefinition   : 
+# 1113|     mu0_2(unknown)   = UnmodeledDefinition : 
+# 1116|     r0_3(glval<int>) = VariableAddress[x]  : 
+# 1116|     mu0_4(int)       = Uninitialized[x]    : &:r0_3
+# 1117|     r0_5(glval<int>) = VariableAddress[y]  : 
+# 1117|     mu0_6(int)       = Uninitialized[y]    : &:r0_5
+# 1118|     r0_7(glval<int>) = VariableAddress[h]  : 
+# 1118|     mu0_8(int)       = Uninitialized[h]    : &:r0_7
+# 1120|     v0_9(void)       = NoOp                : 
+# 1113|     v0_10(void)      = ReturnVoid          : 
+# 1113|     v0_11(void)      = UnmodeledUse        : mu*
+# 1113|     v0_12(void)      = ExitFunction        :