[Testing] TestAST, a helper for writing straight-line AST tests

Tests that need ASTs have to deal with the awkward control flow of
FrontendAction in some way. There are a few idioms used:
 - don't bother with unit tests, use clang -dump-ast
 - create an ASTConsumer by hand, which is bulky
 - use ASTMatchFinder - works pretty well if matchers are actually
   needed, very strange if they are not
 - use ASTUnit - this yields nice straight-line code, but ASTUnit is a
   terrifically complicated library not designed for this purpose

TestAST provides a very simple way to write straight-line tests: specify
the code/flags and it provides an AST that is kept alive until the
object is destroyed.
It's loosely modeled after TestTU in clangd, which we've successfully
used for a variety of tests.

I've updated a couple of clang tests to use this helper, IMO they're clearer.

Differential Revision: https://reviews.llvm.org/D123668