[Index] Return SourceLocation to consumers, not FileID/Offset pair.

Summary:
The FileID/Offset conversion is lossy. The code takes the fileLoc, which loses
e.g. the spelling location in some macro cases.
Instead, pass the original SourceLocation which preserves all information, and
update consumers to match current behavior.

This allows us to fix two bugs in clangd that need the spelling location.

Reviewers: akyrtzi, arphaman

Subscribers: ilya-biryukov, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D45014

llvm-svn: 329570