[clang-tidy/google-readability-casting] Disable check for Objective-C++

Summary:
Previously, `google-readability-casting` was disabled for Objective-C.

The Google Objective-C++ style allows both Objective-C and
C++ style in the same file. Since clang-tidy doesn't have a good
way to allow multiple styles per file, this disables the
check for Objective-C++.

Test Plan: New tests added. Ran tests with:
  % make -j16 check-clang-tools
  Before diff, confirmed tests failed:
  https://reviews.llvm.org/P8081
  After diff, confirrmed tests passed.

Reviewers: alexfh, Wizard, hokein, stephanemoore

Reviewed By: alexfh, Wizard, stephanemoore

Subscribers: stephanemoore, cfe-commits, bkramer, klimek

Differential Revision: https://reviews.llvm.org/D46659

llvm-svn: 332516