[Ada] Missing discriminant checks when accessing variant field

In some cases, the compiler would incorrectly fail to generate
discriminant checks when accessing fields declared in a variant part.
Correct some such cases; detect the remaining cases and flag them as
unsupported. The formerly-problematic cases that are now handled
correctly involve component references occurring in a predicate
expression (e.g., the expression of a Dynamic_Predicate aspect
specification) for a type declaration (not for a subtype declaration).
The cases which are now flagged as unsupported involve expression
functions declared before the discriminated type in question has been
frozen.

gcc/ada/

	* exp_ch3.ads: Replace visible Build_Discr_Checking_Funcs (which
	did not need to be visible - it was not referenced outside this
	package) with Build_Or_Copy_Discr_Checking_Funcs.
	* exp_ch3.adb: Refactor existing code into 3 procedures -
	Build_Discr_Checking_Funcs, Copy_Discr_Checking_Funcs, and
	Build_Or_Copy_Discr_Checking_Funcs. This refactoring is intended
	to be semantics-preserving.
	* exp_ch4.adb (Expand_N_Selected_Component): Detect case where a
	call should be generated to the Discriminant_Checking_Func for
	the component in question, but that subprogram does not yet
	exist.
	* sem_ch13.adb (Freeze_Entity_Checks): Immediately before
	calling Build_Predicate_Function, add a call to
	Exp_Ch3.Build_Or_Copy_Discr_Checking_Funcs in order to ensure
	that Discriminant_Checking_Func attributes are already set when
	Build_Predicate_Function is called.
	* sem_ch6.adb (Analyze_Expression_Function): If the expression
	of a static expression function has been transformed into an
	N_Raise_xxx_Error node, then we need to copy the original
	expression in order to check the requirement that the expression
	must be a potentially static expression. We also want to set
	aside a copy the untransformed expression for later use in
	checking calls to the expression function via
	Inline_Static_Function_Call.  So introduce a new function,
	Make_Expr_Copy, for use in these situations.
	* sem_res.adb (Preanalyze_And_Resolve): When analyzing certain
	expressions (e.g., a default parameter expression in a
	subprogram declaration) we want to suppress checks. However, we
	do not want to suppress checks for the expression of an
	expression function.