gdb/testsuite: fix gdb.cp/converts.exp to run with clang

Clang attempts to minimize the size of the debug-info by not adding
complete information about types that aren't constructed in a given
file.  Specifically, this meant that there was minimal information about
class B in the test gdb.cp/converts.exp.  To fix this, we just need to
construct any object of type B in that file.

Approved-By: Andrew Burgess <aburgess@redhat.com>