gdb/testsuite: add XFAIL to gdb.cp/ptype-flags.exp when using clang

When running gdb.cp/ptype-flags.exp using Clang, we get an unexpected
failure when printing the type of a class with an internal typedef. This
happens because Clang doesn't add accessibility information for typedefs
inside classes (see https://github.com/llvm/llvm-project/issues/57608
for more info). To help with Clang testing, an XFAIL was added to this
test.