[clangd] Use FileEntryRef for canonicalizing filepaths.

Using FileEntry for retrieving filenames give the name used for the last access. FileEntryRef gives the first access name.

Last access name is suspected to change with unrelated changes in clang or the underlying filesystem. First access name gives more stability to the name and makes it easier to track.

Differential Revision: https://reviews.llvm.org/D148213