[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

llvm-svn: 284016
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 9f50fffc0487b74b1828afc4f63d2a8df586df51
diff --git a/include/new b/include/new
index 16b3011..e77d3e6 100644
--- a/include/new
+++ b/include/new
@@ -125,7 +125,7 @@
 
 }  // std
 
-#if defined(_LIBCPP_MSVCRT) && !defined(_LIBCPP_BUILDING_LIBRARY)
+#if defined(_WIN32) && !defined(_LIBCPP_BUILDING_LIBRARY)
 # define _LIBCPP_NEW_DELETE_VIS
 #else
 # define _LIBCPP_NEW_DELETE_VIS _LIBCPP_FUNC_VIS