Eliminate MultipleIncludeOpt::ReadDirective and all calls to it. Any directives

that are lexed are made up of tokens, so the calls are just ugly and redundant.

Hook up the MIOpt for the #if case.  PPCExpressions doesn't currently implement
the hook though, so we still don't handle #if !defined(X) with the MIOpt.

llvm-svn: 38649