gdb/testsuite: disable some tests of gdb.cp/typeid.exp when using Clang

Since Clang chooses to not add any debug information for base types,
expecting it to be included with libraries' informations, gdb.cp/typeid.exp
will always fail if the program hasn't started.  This commit fixes that by
making it so when using Clang, the base type variables aren't tested.

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