[clang] add tests to ExprMutAnalyzer that reproduced a crash in ASTMatchers

[JonasToth]

Summary

This patch adds two unit-tests that are the result of reducing a crashing TU
when running ExprMutAnalyzer over it. They are added only to ensure the regression
that has been fixed with https://reviews.llvm.org/D56444 don't creep back.

[sammccall]

unittests/Analysis/ExprMutationAnalyzerTest.cpp

1112

As I understand, the tests are for the same bug - can we just keep the smaller one?

[JonasToth]

• use only the small reproducer

unittests/Analysis/ExprMutationAnalyzerTest.cpp

TEST(ExprMutationAnalyzerTest, UniquePtr) {
 
   AST = buildASTFromCodeWithArgs(
       UniquePtrDef + "template <class T> void f() { UniquePtr<T> x; x->mf(); }",
       {"-fno-delayed-template-parsing"});
   Results = match(withEnclosingCompound(declRefTo("x")), AST->getASTContext());
   EXPECT_THAT(mutatedBy(Results, AST.get()), ElementsAre("x->mf()"));
 }
 
+TEST(ExprMutationAnalyzerTest, ReproduceFailureMinimal) {
+  const std::string Reproducer =
+      "namespace std {"
+      "template <class T> T forward(T & A) { return static_cast<T&&>(A); }"
+      "template <class T> struct __bind {"
+      "  T f;"
+      "  template <class V> __bind(T v, V &&) : f(forward(v)) {}"
+      "};"
+      "}"
+      "void f() {"
+      "  int x = 42;"
+      "  auto Lambda = [] {};"
+      "  std::__bind<decltype(Lambda)>(Lambda, x);"
+      "}";
+  auto AST11 = buildASTFromCodeWithArgs(Reproducer, {"-std=c++11"});
+  auto Results11 =
+      match(withEnclosingCompound(declRefTo("x")), AST11->getASTContext());
+  EXPECT_FALSE(isMutated(Results11, AST11.get()));
+}
 } // namespace clang