[Syntax] Do not glue multiple empty PP expansions to a single mapping

Summary:
This change makes sure we have a single mapping for each macro expansion,
even if the result of expansion was empty.

To achieve that, we take information from PPCallbacks::MacroExpands into
account. Previously we relied only on source locations of expanded tokens.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62953

llvm-svn: 364236