[clang][clangd] Use reverse header map lookup in suggestPathToFileForDiagnostics

Summary:
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

Reviewers: dexonsmith, bruno, rsmith

Subscribers: cfe-commits

Tasks:

Tags: #clang

Differential Revision: https://reviews.llvm.org/D103142