C++: Improve ReturnStackAllocatedMemory query

Update the ReturnStackAllocatedMmeory query to not give a false positive
for thread_local variables.

Author: Matthew Gretton-Dann

cpp/ql/src/Likely Bugs/Memory Management/ReturnStackAllocatedMemory.ql

@@ -45,6 +45,7 @@ predicate hasNontrivialConversion(Expr e) {
 from LocalScopeVariable var, VariableAccess va, ReturnStmt r
 where
   not var.isStatic() and
+  not var.isThreadLocal() and
   not var.getUnspecifiedType() instanceof ReferenceType and
   not r.isFromUninstantiatedTemplate(_) and
   va = var.getAnAccess() and

cpp/ql/test/query-tests/Likely Bugs/Memory Management/ReturnStackAllocatedMemory/ReturnStackAllocatedMemory.expected

@@ -5,5 +5,4 @@
 | test.cpp:92:2:92:12 | return ... | May return stack-allocated memory from $@. | test.cpp:89:10:89:11 | mc | mc |
 | test.cpp:112:2:112:12 | return ... | May return stack-allocated memory from $@. | test.cpp:112:9:112:11 | arr | arr |
 | test.cpp:119:2:119:19 | return ... | May return stack-allocated memory from $@. | test.cpp:119:11:119:13 | arr | arr |
-| test.cpp:149:3:149:22 | return ... | May return stack-allocated memory from $@. | test.cpp:149:11:149:21 | threadLocal | threadLocal |
 | test.cpp:171:3:171:24 | return ... | May return stack-allocated memory from $@. | test.cpp:170:35:170:41 | myLocal | myLocal |

cpp/ql/test/query-tests/Likely Bugs/Memory Management/ReturnStackAllocatedMemory/test.cpp

@@ -146,7 +146,7 @@ char *testArray5()
 
 int *returnThreadLocal() {
   thread_local int threadLocal;
-  return &threadLocal; // GOOD [FALSE POSITIVE]
+  return &threadLocal; // GOOD
 }
 
 int returnDereferenced() {