Un-XFAIL coverage_no_integrated_as.c test on Windows

You can't use -fno-integrated-as for *-msvc triples because no usable
standalone assembler exists. Perhaps we could teach clang to emit a .s
and then reinvoke itself, but that's a bit silly.

Anyway, fix the test by using an Itanium ABI triple, which will become
mingw, which will assume gnu as is a usable assembler.

llvm-svn: 372994