ada: Parsing error on declare expr in expression default for generic formal function

The compiler incorrectly issues syntax errors for a generic formal function
with default given by an expression (a GNAT extension) when the expression
is a declare_expression.  This is fixed by calling P_Expression_If_OK rather
than P_Expression to parse the expression, which properly handles parsing
declare_expressions without parentheses (but still requires parentheses to
enclose the expression).

gcc/ada/ChangeLog:

	* par.adb (P_Expression_If_OK): Add declare expression to spec comment.
	* par-ch4.adb (P_Expression_If_OK): Add declare expression to comments.
	* par-ch12.adb (P_Formal_Subprogram_Declaration): Call
	P_Expression_If_OK rather than P_Expression.