Improve test infrastructure in SyntaxTree

Summary:
* Test if the code sourcing the SyntaxTree compiles
* Output compiler errors and warnings to err
* Fix tests with code that did not compile

Reviewers: gribozavr2

Reviewed By: gribozavr2

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80731