[libc++] Do not define _LIBCPP_CLANG_VER for non-LLVM Clang

In r292833, we started defining _LIBCPP_CLANG_VER to 0 for Apple Clang.
The result is that AppleClang is detected as being a very old version
of LLVM Clang (version 0), which is obviously incorrect.

I believe this was added so that we don't have to check whether
_LIBCPP_CLANG_VER is defined prior to comparing it with a number
(which can trigger a warning). This commit also fixes the two
places that use the macro correspondingly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@368880 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed