Replace WIN32 define with _WIN32

Only MinGW gcc defines WIN32, MSVC compiler does not. It's
also defined by qmake (msvc-desktop.conf), but not by qbs ...

Let's just use _WIN32, that's defined everywhere.

Change-Id: I8342a70498be54a965dcf7fae63eaf406aaa3c04
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/tools/clangrefactoringbackend/source/clangrefactoringbackend_global.h

@@ -35,7 +35,7 @@ namespace ClangBackEnd {
 using USRName = llvm::SmallVector<char, 128>;
 
 // use std::filesystem::path if it is supported by all compilers
-#ifdef WIN32
+#ifdef _WIN32
 const char nativeSeperator = '\\';
 #else
 const char nativeSeperator = '/';