[runtimes] Officially deprecate the legacy testing configuration system
Add a warning and tweak the release note to explain that the deprecation
targets libc++, libc++abi and libuwnind as well.
Also, as a fly-by, ensure that our CI runs the legacy testing configuration
for libc++, libc++abi and libunwind. This doesn't matter too much since
it's deprecated, but we might as well test it properly.
Differential Revision: https://reviews.llvm.org/D126478
GitOrigin-RevId: 719bf2d9d9fdb8792b15d93364c9c6b2ddd68a36
1 file changed
