[clangd] Catch an unchecked "Expected<T>" in HeaderSourceSwitch.

Summary: Also fixes a potential user-after-scope issue of "Path".

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68564

llvm-svn: 373897