Reorganize predefined macros for all Windows targets.

This adds an option to set the _MSC_VER macro without
recompiling. This is very useful when testing compatibility
with the Windows SDK and c++stdlib headers.

-fmsc-version=<version> (defaults to VS2003 (1300))

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@116999 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/tools/clang.pod b/docs/tools/clang.pod
index b99eb8f..9f7a448 100644
--- a/docs/tools/clang.pod
+++ b/docs/tools/clang.pod
@@ -175,6 +175,10 @@
 
 Enable support for Microsoft extensions.
 
+=item B<-fmsc-version=>
+
+Set _MSC_VER. Defaults to 1300 on Windows. Not set otherwise.
+
 =item B<-fborland-extensions>
 
 Enable support for Borland extensions.