This pr refactors most tests that use RAV to use DRAV instead; this also
has the nice effect of testing both the RAV and DRAV implementations at
the same time w/o having to duplicate all of our AST visitor tests.
Some tests rely on features that DRAV doesn’t support (mainly post-order
traversal), so those haven’t been migrated. At the same time,
`TestVisitor` is now a DRAV, so I’ve had to introduce a new
`CTRPTestVisitor` for any tests that need to use RAV directly.
Some files still contained the old University of Illinois Open Source
Licence header. This patch replaces that with the Apache 2 with LLVM
Exception licence.
Differential Revision: https://reviews.llvm.org/D107528
