[Ada] Implement 'Valid_Value attribute

gcc/ada/

	* libgnat/s-valuen.ads, libgnat/s-valuen.adb
	(Value_Enumeration_Pos): New function to compute the 'Pos of the
	enumeration literal for a given String.  Return a special value
	instead of raising an exception on invalid input. Called by both
	Valid_Enumeration_Image and Value_Enumeration.
	(Valid_Enumeration_Image): Return a Boolean indicating whether
	the String is a valid Image for the given enumeration type.
	(Value_Enumeration): Implement in terms of
	Value_Enumeration_Pos.
	* libgnat/s-vaenu8.ads, libgnat/s-vaen16.ads,
	libgnat/s-vaen32.ads: Rename Valid_Enumeration_Image from the
	instances.
	* libgnat/s-valuti.ads: Correct documentation (it was not true
	for the null string).
	* libgnat/s-valuti.adb (Normalize_String): Do not raise
	Constraint_Error for the null string, nor strings containing
	nothing but blanks, so that Valid_Enumeration_Image can return
	False in these cases, rather than raising an exception.
	* rtsfind.ads (RE_Value_Enumeration_8, RE_Value_Enumeration_16,
	RE_Value_Enumeration_32): New functions.
	(RTE_Available): Improve comment (E doesn't have to be a
	subprogram, although that's the usual case).
	* sem_attr.adb (nalid_Value): Semantic analysis for new
	attribute.
	* exp_attr.adb: Call Expand_Valid_Value_Attribute for new
	attribute.
	* exp_imgv.ads, exp_imgv.adb (Expand_Valid_Value_Attribute): New
	procedure to expand Valid_Value into a call to
	Valid_Enumeration_Image_NN.
	(Expand_Value_Attribute): Misc code cleanups.  Remove two ???
	mark comments. RTE_Available won't work here.  For one thing,
	RTE_Available (X) shouldn't be called until the compiler has
	decided to make use of X (see comments on RTE_Available), and in
	this case we're trying to AVOID calling something.
	* snames.ads-tmpl: New attribute name.
	* doc/gnat_rm/implementation_defined_attributes.rst: Document
	new attribute.
	* gnat_rm.texi: Regenerate.

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