[libc++][test] <=> now has a feature-test macro

...which `test/support/test_macros.h` can use to detect compiler support.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374722 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed