ada: Suppress warnings on derived True/False

ada: Suppress warnings on derived True/False

GNAT normally warns on "return ...;" if the "..." is known to be True or
False, but not when it is a Boolean literal True or False. This patch
also suppresses the warning when the type is derived from Boolean, and
has convention C or Fortran (and therefore True is represented as
"nonzero").

Without this fix, GNAT would give warnings like "False is always False".

gcc/ada/

	* sem_warn.adb
	(Check_For_Warnings): Remove unnecessary exception handler.
	(Warn_On_Known_Condition): Suppress warning when we detect a True
	or False that has been turned into a more complex expression
	because True is represented as "nonzero". (Note that the complex
	expression will subsequently be constant-folded to a Boolean True
	or False). Also simplify to always print "condition is always ..."
	instead of special-casing object names. The special case was
	unhelpful, and indeed wrong when the expression is a literal.