[clangd] Fallback to OrigD when SLoc is invalid

Summary:
Some implicit/built-in decls lack the source location
information. Fallback to OrigD that we've seen in the source code
instead of the canonical one in those cases.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60689

llvm-svn: 358413