Switch from XFAIL to UNSUPPORTED; NFC

This test is currently marked as XFAIL for Windows, but running the
test with a debug build of clang-repl.exe crashes with a modal system
dialog. This switches the test to UNSUPPORTED instead. This makes the
test behavior less onerous for those of us doing Debug builds, at the
expense of a minor bit of coverage if the test were ever to start
passing unexpectedly on Windows (which seems like an unlikely event).

clang/test/Interpreter/execute-weak.cpp

@@ -2,9 +2,8 @@
 // RUN: clang-repl "int i = 10;" 'extern "C" int printf(const char*,...);' \
 // RUN:            'auto r1 = printf("i = %d\n", i);' | FileCheck --check-prefix=CHECK-DRIVER %s
 // REQUIRES: host-supports-jit
-// UNSUPPORTED: system-aix
-// XFAIL: system-windows
 // CHECK-DRIVER: i = 10
+// UNSUPPORTED: system-aix, system-windows
 // RUN: cat %s | clang-repl | FileCheck %s
 extern "C" int printf(const char *, ...);
 int __attribute__((weak)) bar() { return 42; }