[clangd] Fix a crash for accessing a null template decl returned by findExplicitReferences.

Summary: Fixes https://github.com/clangd/clangd/issues/347.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78626

(cherry picked from commit 7d1ee639)
Includes some test-only changes from f651c402
to support the cherry-picked tests.
Test tweaked slightly as it exhibits a separate bug that was fixed on master.