[clangd] Use stable keys for CanonicalIncludes mappings

This patch switches CanonicalInclude mappings to use `llvm::sys::fs::UniqueID` for a stable file representation because the `FileEntry::getName()` results turn out to be changing throughout the lifetime of a program (exposed in D120306). This patch makes it possible for D120306 to be re-landed and increases overall stability.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D123031