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/sourcelocationsutils.h

@@ -58,7 +58,7 @@ Utils::SmallString fromNativePath(Container container)
 {
     Utils::SmallString path(container.data(), container.size());
 
-#ifdef WIN32
+#ifdef _WIN32
     std::replace(path.begin(), path.end(), '\\', '/');
 #endif