gdb/testsuite: ignore Non-C-typedefs for gdb.cp/class2.exp

gdb/testsuite: ignore Non-C-typedefs for gdb.cp/class2.exp

When attempting to test gdb.cp/class2.exp using Clang, it fails to
prepare with the following error:

Executing on host: clang++    -fdiagnostics-color=never -Wno-unknown-warning-option  -c -g -o /home/blarsen/Documents/fsf_build/gdb/testsuite/outputs/gdb.cp/class2/class20.o /home/blarsen/Documents/fsf_build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.cp/class2.cc    (timeout = 300)
builtin_spawn -ignore SIGHUP clang++ -fdiagnostics-color=never -Wno-unknown-warning-option -c -g -o /home/blarsen/Documents/fsf_build/gdb/testsuite/outputs/gdb.cp/class2/class20.o /home/blarsen/Documents/fsf_build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.cp/class2.cc
/home/blarsen/Documents/fsf_build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.cp/class2.cc:53:14: warning: anonymous non-C-compatible type given name for linkage purposes by typedef declaration; add a tag name here [-Wnon-c-typedef-for-linkage]
typedef class {
             ^
              Dbase
/home/blarsen/Documents/fsf_build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.cp/class2.cc:54:2: note: type is not C-compatible due to this member declaration
 public:
 ^~~~~~~
/home/blarsen/Documents/fsf_build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.cp/class2.cc:58:3: note: type is given name 'Dbase' for linkage purposes by this typedef declaration
} Dbase;
  ^
1 warning generated.
gdb compile failed, /home/blarsen/Documents/fsf_build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.cp/class2.cc:53:14: warning: anonymous non-C-compatible type given name for linkage purposes by typedef declaration; add a tag name here [-Wnon-c-typedef-for-linkage]
typedef class {
             ^
              Dbase
/home/blarsen/Documents/fsf_build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.cp/class2.cc:54:2: note: type is not C-compatible due to this member declaration
 public:
 ^~~~~~~
/home/blarsen/Documents/fsf_build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.cp/class2.cc:58:3: note: type is given name 'Dbase' for linkage purposes by this typedef declaration
} Dbase;
  ^
1 warning generated.
UNTESTED: gdb.cp/class2.exp: failed to prepare

This can be silenced by adding -Wno-non-c-typedef-for-linkage for Clang
11 or later.  The test shows no failures with this change.

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