Distinguish between library and language support for aligned allocation.

There are two cases:
1. The library has all it needs to provide align_val_t and the
new/delete overloads needed to support aligned allocation.
2. The compiler has actually turned the language feature on.

There are times where libc++ needs to distinguish between the two.

This patch adds the additional macro
_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION which denotes when case (1)
does not hold. _LIBCPP_HAS_NO_ALIGNED_ALLOCATION is defined whenever
_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION is defined, or when the
compiler has not enabled the language feature.

Additionally this patch cleans up a number of other macros related
to detection of aligned allocation machinery.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@344207 91177308-0d34-0410-b5e6-96231b3b80d8

include/memory

@@ -2016,7 +2016,7 @@ get_temporary_buffer(ptrdiff_t __n) _NOEXCEPT
         __n = __m;
     while (__n > 0)
     {
-#if !defined(_LIBCPP_HAS_NO_ALIGNED_ALLOCATION)
+#if !defined(_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION)
     if (__is_overaligned_for_new(__alignof(_Tp)))
         {
             std::align_val_t __al =