[libclang] Get rid of relience on SourceManager member signature

Summary:
Libclang was enforcing a singature on SourceManager::getLocalSLocEntry
which isn't possible to satisfy as pointed out in
https://reviews.llvm.org/D80681#inline-751438.

This patch updates the libclang, hopefully without breaking API stability, to
not rely on member signature. To enable changing the signature in D82498.

Reviewers: sammccall, bkramer

Subscribers: arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82532