PTH: use a variable instead of a macro

Cleanup only.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@212457 91177308-0d34-0410-b5e6-96231b3b80d8
PTHInternals.rst: PCH is now a thing, update docs to reflect it

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@212458 91177308-0d34-0410-b5e6-96231b3b80d8