[SemaCodeComplete] Expose a method to create CodeCompletionString for macros.

Summary:
The method only takes PPreprocessor and don't require structures that
might not be available (e.g. Sema and ASTContext) when CodeCompletionString
needs to be generated for macros.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48973

llvm-svn: 336427