suggestPathToFileForDiagnostics is actively used in clangd for converting
an absolute path to a header file to a header name as it should be spelled
in the sources. Current approach converts absolute path to relative path.
This diff implements missing logic that makes a reverse lookup from the
relative path to the key in the header map that should be used in the sources.
Prerequisite diff https://reviews.llvm.org/D103229
Test Plan: check-clang