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

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

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@374722 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx