[Clangd] textDocument/definition and textDocument/declaration "bounce" between...

[Clangd] textDocument/definition and textDocument/declaration "bounce" between definition and declaration location when they are distinct.

Summary:
This helps minimize the disruption of not returning declarations as part of
a find-definition response (r352864).

Reviewers: hokein

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, ilya-biryukov

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57580

llvm-svn: 352953