c++: P1937R2 - Fixing inconsistencies between const{expr,eval} functions

c++: P1937R2 - Fixing inconsistencies between const{expr,eval} functions

The following patch implements my understanding of P1937R2, though I wonder
if https://eel.is/c++draft/expr.const#14.example-1 shouldn't have been
also either removed or adjusted by the P1937R2 paper.

2020-02-24  Jakub Jelinek  <jakub@redhat.com>

	P1937R2 - Fixing inconsistencies between const{expr,eval} functions
	* call.c (build_over_call): Don't evaluate immediate functions in
	unevaluated operands.

	* g++.dg/ext/consteval1.C: Change dg-{message,error} into dg-bogus.
	* g++.dg/cpp2a/consteval6.C: Likewise.
	* g++.dg/cpp2a/consteval3.C: Change dg-error for unevaluated operands
	into dg-bogus.