gdb/testsuite: add XFAIL to gdb.cp/derivation.exp when using Clang

When running gdb.cp/derivation.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.

Approved-by: Tom Tromey <tom@tromey.com>