[lldb] Simplify the logic to detect compiler flag support

This patch effectively does the following 3 things:

 - Centralize the logic to figure out if a compiler flag is supported.
 - Stop sanity checking whether the compiler works at all. While useful,
   that's not the decorator's responsibility.
 - Invoke the compiler with xcrun on Darwin so we know where to find the
   sysroot.

On my macOS Big Sur system, the clang invocation couldn't find libSystem
and would fail the sanity check in the decorator. This meant that the
test suite would always try to run the ASan/UBSan/TSan tests, regardless
of whether compiler-rt was built.

Differential revision: https://reviews.llvm.org/D95995