Nico Rieck:  Currently _MSC_VER and _WIN32 are used to guard code which is
MSVC-specific, MSVCRT-specific, or Windows-specific. Because Clang can
also define _MSC_VER, and MSVCRT is not necessarily the only C runtime,
these macros should not be used interchangeably.

This patch divides all Windows-related bits into the aforementioned
categories. Two new macros are introduced:

- _LIBCPP_MSVC: Defined when compiling with MSVC. Detected using
  _MSC_VER, excluding Clang.
- _LIBCPP_MSVCRT: Defined when using the Microsoft CRT. This is the default
   when _WIN32 is defined.

This leaves _WIN32 for code using the Windows API.

This also corrects the spelling of _LIBCP_HAS_IS_BASE_OF to _LIBCPP_HAS_IS_BASE_OF.

Nico, please prepare a patch for CREDITS.TXT, thanks.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@187593 91177308-0d34-0410-b5e6-96231b3b80d8