c++: Handle bad pack expansion in base list. [PR96752]

c++: Handle bad pack expansion in base list. [PR96752]

This fixes PR95672 by adding the missing TYPE_PACK_EXPANSION case in
cxx_incomplete_type_diagnostic in order to avoid ICEs on diagnosing
incomplete template pack expansion cases.

Tested on powerpc64le-unknown-linux-gnu.

gcc/cp/ChangeLog:

	PR c++/95672
	* typeck2.c (cxx_incomplete_type_diagnostic): Add missing
	TYPE_EXPANSION_PACK check for diagnosing incomplete types in
	cxx_incomplete_type_diagnostic.

gcc/testsuite/ChangeLog:

	PR c++/95672
	* g++.dg/template/pr95672.C: New test.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>