c++: Fix unchecked use of CLASSTYPE_AS_BASE [PR113031]

c++: Fix unchecked use of CLASSTYPE_AS_BASE [PR113031]

My previous commit (naively) assumed that a TREE_CODE of RECORD_TYPE or
UNION_TYPE was sufficient for optype to be considered a "class type".
However, this does not account for e.g. template type parameters of
record or union type. This patch corrects to check for CLASS_TYPE_P
before checking for as-base conversion.

	PR c++/113031

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_fold_indirect_ref_1): Check for CLASS_TYPE
	before using CLASSTYPE_AS_BASE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/pr113031.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>