Extend loop variable naming checks

The loop convert tests for conflicting names have been extended to check for
macro names, types, and language keywords including language extensions. Tests
have also been added.

Fixes PR15322

Author: Jack Yang <jack.yang@intel.com>
Reviewer: gribozavr, klimek, revane


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@176690 91177308-0d34-0410-b5e6-96231b3b80d8