c++: Further tweak for P1937R2 - const{expr,eval} inconsistencies

c++: Further tweak for P1937R2 - const{expr,eval} inconsistencies

Seems I've missed one thing, as the first hunk in
https://github.com/cplusplus/draft/commit/c8e68ed202b4a9260616bcee8a9768b5dca4bbca
changes the wording so that only potentially-evaluated id-expressions that
denote immediate functions must appear only in the specified contexts.
That IMO means that in unevaluated contexts there aren't such restrictions
anymore, so I think in unevaluated contexts one should be able to take the
address of an immediate function.

2020-02-28  Jakub Jelinek  <jakub@redhat.com>

	P1937R2 - Fixing inconsistencies between const{expr,eval} functions
	* typeck.c (cp_build_addr_expr_1): Allow taking address of immediate
	functions in unevaluated contexts.

	* g++.dg/cpp2a/consteval3.C: Change dg-error about taking address of
	immediate function in unevaluated contexts into dg-bogus.
	* g++.dg/cpp2a/consteval16.C: New test.