tests: enable a couple of C++ value witness table tests

This enables the IRGen test and marks the dtor tests as XFAIL.  Two of
the tests will currently abort (likely due to invalid CodeGen), so mark
the test as XFAIL to at least get build coverage.

Author: compnerd

test/Interop/Cxx/value-witness-table/custom-destructors-virtual-irgen.swift

@@ -3,9 +3,6 @@
 // FIXME: Once we can link with libc++ we can start using RTTI.
 //
 // RUN: %target-swift-frontend -enable-experimental-cxx-interop -I %S/Inputs %s -emit-ir -Xcc -fno-rtti | %FileCheck %s
-//
-// Windows doesn't support -fno-rtti.
-// UNSUPPORTED: OS=windows-msvc
 
 import CustomDestructor
 

test/Interop/Cxx/value-witness-table/custom-destructors-virtual.swift

@@ -6,8 +6,7 @@
 // RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -Xcc -fno-rtti -O)
 //
 // REQUIRES: executable_test
-// Windows doesn't support -fno-rtti.
-// UNSUPPORTED: OS=windows-msvc
+// XFAIL: OS=windows-msvc
 
 import CustomDestructor
 import StdlibUnittest