One of the big limitations we have in the compiler-rt build system today is that we cannot bootstrap building the builtins because you need a fully functional toolchain to pass CMake's tests.
This change adds support for compile only tests.
It is NFC because nothing is using the compile-only tests yet.
I believe this is the last separable part of D16653.
Trusting the return code of the compiler is a bit naive. Upstream cmake tests all of the following messages to find unsupported flags:
We don't support most of those compilers, but we do support MSVC, which ignores unknown flags, so we need to do something here.