[libcxx] [test] Convert an XFAIL: LIBCXX-WINDOWS-FIXME into XFAIL: msvc with explanation

This test doesn't fail in mingw mode (which uses the same Itanium
name mangling and ABI as other platforms).

Differential Revision: https://reviews.llvm.org/D112210

Author: mstorsjo

libcxx/test/libcxx/language.support/support.rtti/type.info/type_info.comparison.merged.sh.cpp

@@ -8,7 +8,8 @@
 
 // UNSUPPORTED: no-rtti
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
+// In MSVC mode, the two anonymous types have identical type index in both object files.
+// XFAIL: msvc
 
 // RUN: %{cxx} %s %{flags} %{compile_flags} -c -o %t.tu1.o -DTU1 -D_LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION=1
 // RUN: %{cxx} %s %{flags} %{compile_flags} -c -o %t.tu2.o -DTU2 -D_LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION=1