[libTooling] Add option for `buildAST` to report diagnostics.

Summary:
Currently, `buildAST[WithArgs]` either succeeds or fails.  This patch adds
support for the caller to pass a `DiagnosticConsumer` to receive all relevant
diagnostics.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74763