[clangd] Adapt index interfaces to D45014, and fix the old bugs.

Summary:
Fix bugs:
- don't count occurrences of decls where we don't spell the name
- findDefinitions at MACRO(^X) goes to the definition of MACRO

Subscribers: klimek, ilya-biryukov, jkorous-apple, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D45356

llvm-svn: 329571