c++: Fix ICE with functional cast to reference in template [PR123044]

c++: Fix ICE with functional cast to reference in template [PR123044]

When processing a functional cast to a reference type in a template
context, build_functional_cast_1 wasn't calling convert_from_reference.
This left the expression with a reference type, which later triggered
an assertion in implicit_conversion from r15-6709 that expects
expression types to already be dereferenced.

In contrast, cp_build_c_cast already calls convert_from_reference on
the result in template contexts, so C-style casts like (R)x worked
correctly.

The fix makes functional casts consistent with C-style casts by
calling convert_from_reference before returning in the template
processing path.

	PR c++/123044

gcc/cp/ChangeLog:

	* typeck2.cc (build_functional_cast_1): Call convert_from_reference
	on template CAST_EXPR to match C-style cast behavior.

gcc/testsuite/ChangeLog:

	* g++.dg/template/implicit-func-cast.C: New test.

Signed-off-by: Egas Ribeiro <egas.g.ribeiro@tecnico.ulisboa.pt>
Reviewed-by: Jason Merrill <jason@redhat.com>