Take an entirely different approach to handling the "parsing" of

Take an entirely different approach to handling the "parsing" of
__import__ within the preprocessor, since the prior one foolishly
assumed that Preprocessor::Lex() was re-entrant. We now handle
__import__ at the top level (only), after macro expansion. This should
fix the buildbot failures.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138704 91177308-0d34-0410-b5e6-96231b3b80d8