[clangd] Prioritize indexing of files that share a basename with the open file.

Summary:
In practice, opening Foo.h will still often result in Foo.cpp making the
second index build instead of the first, as the rebuild policy doesn't
know to wait.

Reviewers: kadircet

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64575

llvm-svn: 365888