c++: CTAD with alias and nested template [PR105655]

c++: CTAD with alias and nested template [PR105655]

Here, alias_ctad_tweaks expect tsubst_decl of a FUNCTION_DECL to return a
FUNCTION_DECL.  A reasonable expectation, but in this case we were replacing
the template args of the class-scope deduction guide with equivalent args,
so looking in the hash table we found the partial instantiation stored when
instantiating A<int>, which is a TEMPLATE_DECL.  It's fine for that to be
what is stored, but tsubst_function_decl should never return it.

	PR c++/105655

gcc/cp/ChangeLog:

	* pt.cc (build_template_decl): Add assert.
	(tsubst_function_decl): Don't return a template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-alias13.C: New test.