[clangd] Prefer location from codegen files when merging symbols.
For example, if an index symbol has location in a .proto file and an AST symbol
has location in a generated .proto.h file, then we prefer location in .proto
which is more meaningful to users.
Also use mergeSymbols to get the preferred location between AST location and index location in go-to-def.
Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D58037