[clangd] Cleanup: use index file instead of header in workspace symbols lit test.

Summary:
The full path of the input header depends on the execution environment
and may result in different behavior (e.g. when different URI schemes are used).

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D54833

llvm-svn: 347466