[PCH] Remove useless UNSUPPORTED clause

"win32" is never defined as a lit feature, AFAICT, and Windows bots
appear to be running this just fine.

Part of the project to eliminate special handling for triples in lit expressions.

clang/test/PCH/modified-header-error.c

@@ -9,6 +9,3 @@
 #include "header2.h"
 
 // CHECK: fatal error: file {{.*}} has been modified since the precompiled header {{.*}} was built
-
-// FIXME: Flaky on Windows, timestamp resolution?
-// UNSUPPORTED: win32