[Ada] Implement AI12-0101

[Ada] Implement AI12-0101

2019-12-13  Steve Baird  <baird@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_N_Op_Eq.Is_Equality): Move this function
	from within Expand_N_Op_Eq.Find_Equality out to immediately
	within Expand_N_Op_Eq in order to give it greater visibility.
	Add a new Typ parameter (defaulted to Empty) which, if
	non-empty, means the function will return False in the case of
	an equality op for some other type.
	* (Expand_N_Op_Eq.User_Defined_Primitive_Equality_Op): A new
	function. Given an untagged record type, finds the corresponding
	user-defined primitive equality op (if any).  May return Empty.
	Ignores visibility.
	* (Expand_N_Op): For Ada2012 or later, check for presence of a
	user-defined primitive equality op before falling back on the
	usual predefined component-by-component comparison. If found,
	then call the user-defined op instead.

From-SVN: r279341