The test failed silently if lld wasn't built alongside clang.
But the test uses -###, so the "invalid linker name in -fuse-ld=lld"
diag didn't make clang fail, and something else happened to match
"-demangle", so the test passed.
To fix, pass -B to a directory with two empty +x files (which works
on non-Windows), and look for "-demangle" instead of just -demangle.
Also force linker_version to 0 and pass a darwin triple.