This might let us get to using toybox grep for the build faster than if
we have to implement PCRE support.
Test: builds
Change-Id: Idaf24cfefbf18fdc442871f24fe4fe9b2152359a
Clang uses this for its own diagnostic tests. The nice feature it adds
here is that it fails if clang emits a diagnostic that we don't expect
(and we get to drop a dependency on FileCheck).
This also tweaks and renames file-check-cxx to reflect its new job.
Bug: 131861088
Test: mma
Change-Id: I77f7ce77869edaa23e2401e622ad7007d2fee06c