s/LLVM_ON_WIN32/_WIN32/, llvm

LLVM_ON_WIN32 is set exactly with MSVC and MinGW (but not Cygwin) in
HandleLLVMOptions.cmake, which is where _WIN32 defined too.  Just use the
default macro instead of a reinvented one.

See thread "Replacing LLVM_ON_WIN32 with just _WIN32" on llvm-dev and cfe-dev.
No intended behavior change.

This moves over all uses of the macro, but doesn't remove the definition
of it in (llvm-)config.h yet.

llvm-svn: 331127
diff --git a/llvm/examples/Kaleidoscope/Chapter9/toy.cpp b/llvm/examples/Kaleidoscope/Chapter9/toy.cpp
index 821cf4d..7a12b5a 100644
--- a/llvm/examples/Kaleidoscope/Chapter9/toy.cpp
+++ b/llvm/examples/Kaleidoscope/Chapter9/toy.cpp
@@ -1379,7 +1379,7 @@
 // "Library" functions that can be "extern'd" from user code.
 //===----------------------------------------------------------------------===//
 
-#ifdef LLVM_ON_WIN32
+#ifdef _WIN32
 #define DLLEXPORT __declspec(dllexport)
 #else
 #define DLLEXPORT