ada: Fix format string parsing in GNAT.Formatted_String

ada: Fix format string parsing in GNAT.Formatted_String

Before this patch, format strings ending with "%%" (two consecutive
percent signs) caused GNAT.Formatted_String."-" to give the wrong
output, and cause the various GNAT.Formatted_String."&" to raise
exceptions with misleading error messages.

Also before this patch, a bug in GNAT.Formatted_String."-" caused
characters from the format string to be dropped. Calling
GNAT.Formatted_String."-" on an instance of
GNAT.Formatted_String.Formatted_String caused subsequent uses of
that instance to return wrong results.

In addition to fixing the parsing of format strings, this patch
centralizes the detection of format specifiers in a unique
procedure.

gcc/ada/

	* libgnat/g-forstr.adb
	(Advance_And_Accumulate_Until_Next_Specifier): New procedure.
	("-"): Replace inline code with call to
	Advance_And_Accumulate_Until_Next_Specifier.
	(Next_Format): likewise.