[clangd] Avoid collecting symbols from broken TUs in global-symbol-builder.

Summary:
For example, template parameter might not be resolved in a broken TU,
which can result in wrong USR/SymbolID.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48881

llvm-svn: 336252