[clangd] Fix diagnostic location for macro expansions

Summary:
Diagnostic locations were broken when it was result of a macro
expansion. This patch fixes it by using expansion location instead of location
inside macro body.

Fixes https://github.com/clangd/clangd/issues/201.

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70494