[Tooling][libclang] Remove unused CompilationDatabase::MappedSources

Summary:
This field is never assigned to and it's only ever read from libclang.
This patch removes it and adapts libclang to return constants.

Reviewers: klimek, bkramer

Reviewed By: klimek

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D32351

llvm-svn: 303635