C++: Don't create partial defs for calls to const

These partial defs don't do any harm, but they could hurt performance.
In typical C++ snapshots, between 5% and 20% of all calls are to `const`
functions.

Author: jbj

cpp/ql/src/semmle/code/cpp/dataflow/internal/FlowVar.qll

@@ -113,7 +113,12 @@ private module PartialDefinitions {
         )
       )
     } or
-    TExplicitCallQualifier(Expr qualifier, Call call) { qualifier = call.getQualifier() } or
+    TExplicitCallQualifier(Expr qualifier) {
+      exists(Call call |
+        qualifier = call.getQualifier() and
+        not call.getTarget().hasSpecifier("const")
+      )
+    } or
     TReferenceArgument(Expr arg, VariableAccess va) { referenceArgument(va, arg) }
 
   private predicate isInstanceFieldWrite(FieldAccess fa, ControlFlowNode node) {
@@ -128,7 +133,7 @@ private module PartialDefinitions {
     PartialDefinition() {
       this = TExplicitFieldStoreQualifier(definedExpr, node)
       or
-      this = TExplicitCallQualifier(definedExpr, _) and node = definedExpr
+      this = TExplicitCallQualifier(definedExpr) and node = definedExpr
       or
       this = TReferenceArgument(definedExpr, node)
     }

cpp/ql/test/library-tests/dataflow/partialdefinitions/partialdefinitions.expected

@@ -3,5 +3,3 @@
 | partialdefinitions.cpp:15:4:15:4 | partial def of y | partialdefinitions.cpp:15:4:15:4 | y | partialdefinitions.cpp:15:2:15:10 | ... = ... |
 | partialdefinitions.cpp:22:29:22:32 | partial def of this | file://:0:0:0:0 | this | partialdefinitions.cpp:22:29:22:40 | ... = ... |
 | partialdefinitions.cpp:27:3:27:7 | partial def of myInt | partialdefinitions.cpp:27:3:27:7 | myInt | partialdefinitions.cpp:27:3:27:7 | myInt |
-| partialdefinitions.cpp:28:7:28:11 | partial def of myInt | partialdefinitions.cpp:28:7:28:11 | myInt | partialdefinitions.cpp:28:7:28:11 | myInt |
-| partialdefinitions.cpp:30:10:30:14 | partial def of myInt | partialdefinitions.cpp:30:10:30:14 | myInt | partialdefinitions.cpp:30:10:30:14 | myInt |