[clang][index] Mark file as C++ in parse-all-comments test

`CompilerInvocation::CreateFromArgs` doesn't always report command line parsing failures through the return value. Sometimes, errors are only reported via diagnostics.

Some clients like `c-index-test` only check the return value and don't check the state of `DiagnosticsEngine`.

If we were to start returning the correct return value from `CreateFromArgs`, this index test starts to fail, because it specifies `-std=c++11` for a C input, which is invalid.

This patch fixes that issue by adding forgotten `-x c++` argument.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D95879