[clang] tests: cleanup, update and add some new ones
This reworks a small set of tests, as preparatory work for implementing
- Run for more standard versions, including c++2b.
- Normalize file names and run commands.
- Adds some extra tests.
New Coroutine tests taken from Aaron Puchert's D68845.
Signed-off-by: Matheus Izvekov <firstname.lastname@example.org>
Reviewed By: thakis
Differential Revision: https://reviews.llvm.org/D99225