Reapply r289181 but rename the include guard to avoid
conflict with the one from Darwin.

Allow darwin to provide additional definitions and implementation
specifc values for tgmath.h on Apple platforms.

rdar://problem/19019845

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298013 91177308-0d34-0410-b5e6-96231b3b80d8