Avoid errors on header guards using #if defined(NAME).

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188306 91177308-0d34-0410-b5e6-96231b3b80d8
Enhanced test for header guards.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@188308 91177308-0d34-0410-b5e6-96231b3b80d8