Add -frtti and -fexceptions to tests that assume these are on.

[filcab]

Summary

We now have targets that don't enable rtti/exceptions by default, and the
ASTMatchers tests are assuming that these features are on (e.g: They use
dynamic_cast or try).

[thakis]

I'm not familiar with this code, but the change makes "obviously sense". I think you can go ahead and land this.

unittests/ASTMatchers/ASTMatchersTest.h

testing::AssertionResult matchesConditionally(
   VerifyMatch VerifyFound(nullptr, &Found);
   Finder.addMatcher(AMatcher, &VerifyFound);
   VerifyMatch VerifyDynamicFound(nullptr, &DynamicFound);
   if (!Finder.addDynamicMatcher(AMatcher, &VerifyDynamicFound))
     return testing::AssertionFailure() << "Could not add dynamic matcher";
   std::unique_ptr<FrontendActionFactory> Factory(
       newFrontendActionFactory(&Finder));
   // Some tests use typeof, which is a gnu extension.
-  std::vector<std::string> Args(1, CompileArg);
+  std::vector<std::string> Args;
+  Args.push_back(CompileArg);
+  // Some tests need rtti/exceptions on
+  Args.push_back("-frtti");
+  Args.push_back("-fexceptions");
   if (!runToolOnCodeWithArgs(Factory->create(), Code, Args, "input.cc",
                              VirtualMappedFiles)) {
     return testing::AssertionFailure() << "Parsing error in \"" << Code << "\"";
   }
   if (Found != DynamicFound) {
     return testing::AssertionFailure() << "Dynamic match result ("
                                        << DynamicFound
                                        << ") does not match static result ("