Migrate the roundtrip tests to work similar to the llgdb tests

I'm not sure if there's a way to make this a bit more general - the
property that matters is that there's /some/ itanium ABI target the
tests can use to compile - not link or run in this case. But this seems
sufficient for the llgdb tests, so it should be sufficient (though
perhaps not necessary) for this roundtrip test.

cross-project-tests/debuginfo-tests/clang_llvm_roundtrip/lit.local.cfg

+# In MSVC mode DWARF isn't produced & is needed for these tests
+if 'native' not in config.available_features or config.is_msvc:
+    config.unsupported = True

cross-project-tests/debuginfo-tests/clang_llvm_roundtrip/simplified_template_names.cpp

-// RUN: %clang %s -c -o - -g -Xclang -gsimple-template-names=mangled -Xclang -debug-forward-template-params \
+// RUN: %clang %target_itanium_abi_host_triple %s -c -o - -g -Xclang -gsimple-template-names=mangled -Xclang -debug-forward-template-params \
 // RUN:   | llvm-dwarfdump --verify -
-// REQUIRES: native
 template<typename ...Ts>
 struct t1 {
 };