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/lib/Support/CodeGenCoverage.cpp b/llvm/lib/Support/CodeGenCoverage.cpp
index 4948f13..4114cfc 100644
--- a/llvm/lib/Support/CodeGenCoverage.cpp
+++ b/llvm/lib/Support/CodeGenCoverage.cpp
@@ -22,7 +22,7 @@
 
 #if LLVM_ON_UNIX
 #include <unistd.h>
-#elif LLVM_ON_WIN32
+#elif _WIN32
 #include <windows.h>
 #endif
 
@@ -93,7 +93,7 @@
     std::string Pid =
 #if LLVM_ON_UNIX
         llvm::to_string(::getpid());
-#elif LLVM_ON_WIN32
+#elif _WIN32
         llvm::to_string(::GetCurrentProcessId());
 #else
         "";