[clangd] Fix "fail to create file URI" warnings in FileIndexTest.

Summary:
When running the FileIndexTest, it shows "Failed to create an URI
for file XXX: not a valid absolute file path" warnings, and the
generated Symbols is missing Location information.

Reviewers: ioeric

Subscribers: klimek, ilya-biryukov, jkorous-apple, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D45692

llvm-svn: 330182