C++: Add tests for three FPs observed on lgtm.com

Author: jbj

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

@@ -5,3 +5,6 @@
 | 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:155:3:155:18 | return ... | May return stack-allocated memory from $@. | test.cpp:154:19:154:26 | localInt | localInt |
+| test.cpp:165:3:165:19 | return ... | May return stack-allocated memory from $@. | test.cpp:164:21:164:28 | localBuf | localBuf |

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

@@ -143,3 +143,24 @@ char *testArray5()
 
 	return arr; // GOOD
 }
+
+int *returnThreadLocal() {
+  thread_local int threadLocal;
+  return &threadLocal; // GOOD [FALSE POSITIVE]
+}
+
+int returnDereferenced() {
+  int localInt = 2;
+  int &localRef = localInt;
+  return localRef; // GOOD [FALSE POSITIVE]
+}
+
+typedef unsigned long size_t;
+void *memcpy(void *s1, const void *s2, size_t n);
+
+char *returnAfterCopy() {
+  char localBuf[] = "Data";
+  static char staticBuf[sizeof(localBuf)];
+  memcpy(staticBuf, localBuf, sizeof(staticBuf));
+  return staticBuf; // GOOD [FALSE POSITIVE]
+}