[libc++] Correct explanation of _LIBCPP_NEW_DELETE_VIS

The behavior of this macro actually needs to apply universally on
Windows and not just when using the Microsoft CRT. Update the macro
definition and documentation accordingly.

Differential Revision: https://reviews.llvm.org/D25145

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@284016 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed