[clangd] Perform merge for main file symbols.

Summary:
Previously, we randomly pick one main file symbol in dynamic index, we
may loose the ideal symbol (with definition location) in the index.

It fixes the issue where sometimes we fail to go to the symbol definition, see:

1. call go-to-decl on Foo in Foo.cpp
2. jump to Foo.h, call go-to-def on Foo in Foo.h

we can't go back to Foo.cpp -- because we open Foo.cpp, Foo.h in clangd, both
files have Foo symbol (one with def&decl, one with decl only), we randomely
choose one.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63425

llvm-svn: 363568