c++: constexpr, inheritance, and local class [PR91933]

Here we complained about referring to nm3 from the local class member
function because referring to the base class subobject involved taking the
variable's address.  Let's shortcut this case to avoid that.

gcc/cp/ChangeLog:

	PR c++/91933
	* class.c (build_base_path): Shortcut simple non-pointer case.

gcc/testsuite/ChangeLog:

	PR c++/91933
	* g++.dg/cpp0x/constexpr-base7.C: New test.