Fix regression in pointer-to-member printing

Fix regression in pointer-to-member printing

PR c++/29243 points out that "info func" on a certain C++ executable
will cause an infinite loop in gdb.

I tracked this down to a bug introduced by commit 6b5a7bc76 ("Handle
member pointers directly in generic_value_print").  Before this
commit, the C++ code to print a member pointer would wind up calling
value_print_scalar_formatted; but afterward it simply calls
generic_value_print and gets into a loop.

This patch restores the previous behavior and adds a regression test.