[gdb/exp] Fix internal error when printing C++ pointer-to-member

When running the test-case included with this patch, we run into:
...
(gdb) print ptm^M
$1 = gdb/gdbtypes.h:695: internal-error: loc_bitpos: \
  Assertion `m_loc_kind == FIELD_LOC_KIND_BITPOS' failed.^M
...
while printing a c++ pointer-to-member.

Fix this by skipping static fields in cp_find_class_member, such that we have:
...
(gdb) print ptm^M
$1 = &A::i^M
...

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29294