[clangd] Rename: merge index/AST refs path-insensitively where needed

If you have c:\foo open, and C:\foo indexed (case difference) then these
need to be considered the same file. Otherwise we emit edits to both,
and editors do... something that isn't pretty.

Maybe more centralized normalization is called for, but it's not trivial
to do this while also being case-preserving. see
https://github.com/clangd/clangd/issues/108

Fixes https://github.com/clangd/clangd/issues/665

Differential Revision: https://reviews.llvm.org/D95759