[clang-tidy ObjC] [2/3] Support non-C++ files in ClangTidyTest

Summary:
This is part 2 of 3 of a series of changes to improve
Objective-C linting in clang-tidy.

Currently, `clang::tidy::test::runCheckOnCode()` assumes all files
are C++ and unconditionally adds `-std=c++11` to the list of
`clang-tidy` options.

This updates the logic to check the extension of the source file
and only add `-std=c++11` if the extension indicates C++ or
Objective-C++.

Depends On D39188

Test Plan:

  ninja ClangTidyTests && \
  ./tools/clang/tools/extra/unittests/clang-tidy/ClangTidyTests

Patch by Ben Hamilton!

Reviewers: hokein, alexfh

Reviewed By: hokein

Subscribers: Wizard

Differential Revision: https://reviews.llvm.org/D39189

llvm-svn: 316645