[testing] Convert XFAIL to UNSUPPORTED in C++ module testing
[testing] Convert XFAIL to UNSUPPORTED in C++ module testing

It's possible to build Swift where libcxx is implicitly used, so this
test is brittle if it uses XFAIL.

benlangmuir/swift,rudkx/swift,gregomni/swift,benlangmuir/swift,atrick/swift,rudkx/swift,ahoppen/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,rudkx/swift,atrick/swift,glessard/swift,atrick/swift,apple/swift,xwu/swift,atrick/swift,JGiola/swift,glessard/swift,roambotics/swift,ahoppen/swift,rudkx/swift,roambotics/swift,ahoppen/swift,ahoppen/swift,gregomni/swift,roambotics/swift,JGiola/swift,roambotics/swift,xwu/swift,glessard/swift,xwu/swift,benlangmuir/swift,rudkx/swift,apple/swift,JGiola/swift,apple/swift,xwu/swift,rudkx/swift,glessard/swift,gregomni/swift,apple/swift,apple/swift,glessard/swift,benlangmuir/swift,roambotics/swift,xwu/swift,ahoppen/swift,atrick/swift,ahoppen/swift,gregomni/swift,gregomni/swift,JGiola/swift,JGiola/swift,gregomni/swift,atrick/swift,glessard/swift,roambotics/swift,xwu/swift,JGiola/swift,xwu/swift