[libTooling] Extend `buildASTFromCodeWithArgs` to take files argument.

Summary:
Adds an optional parameter to `buildASTFromCodeWithArgs` that allows the user to
pass additional files that the main code needs to compile. This change makes
`buildASTFromCodeWithArgs` consistent with `runToolOnCodeWithArgs`.

Patch by Alexey Eremin.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70175