[Ada] Allow formal functions to have a default in the form of an expression function

gcc/ada/

	* doc/gnat_rm/implementation_defined_pragmas.rst: Add
	documentation of the new form of formal subprogram default in
	the section on language extensions (pragma Extensions_Allowed).
	* gnat_rm.texi: Regenerate.
	* gen_il-gen-gen_nodes.adb: Add Expression as a syntactic field
	of N_Formal_(Abstract|Concrete)_Subprogram_Declaration nodes.
	* par-ch12.adb (P_Formal_Subprogram_Declaration): Add parsing
	support for the new default of a parenthesized expression for
	formal functions. Issue an error when extensions are not
	allowed, suggesting use of -gnatX. Update comment with extended
	syntax for SUBPROGRAM_DEFAULT.
	* sem_ch12.adb (Analyze_Formal_Subprogram_Declaration): Issue an
	error when an expression default is given for an abstract formal
	function. When a default expression is present for a formal
	function, install the function's formals and preanalyze the
	expression.
	(Instantiate_Formal_Subprogram): Fix typo in RM paragraph in a
	comment.  When a formal function has a default expression,
	create a body for the function that will evaluate the expression
	and will be called when the default applies in an instantiation.
	The implicit function is marked as inlined and as having
	convention Intrinsic.

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