[clangd] Also perform merging for symbol definitions

Summary:
clangd currently prefers declarations from codegen files. This patch
implements that behavior for definition locations. If we have definiton
locations both coming from AST and index, clangd will perform a merging to show
the codegen file if that's the case.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61126

llvm-svn: 359874