[clangd] Assert that the testcases in LocateSymbol.All have no diagnostics

Summary: Also fix some bugs in the testcases which this exposed.

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72066