[Ada] gnatxref: infinite loop on symbols not found

[Ada] gnatxref: infinite loop on symbols not found

This patch fixes a bug in which if a symbol is not found, gnatxref can
sometimes enter an infinite loop. No impact on compilation.

2019-09-19  Bob Duff  <duff@adacore.com>

gcc/ada/

	* xref_lib.adb (Get_Symbol_Name): If we reach EOF in the first
	loop without finding the symbol, return "???". Otherwise, it's
	an infinite loop.
	(Parse_EOL): Assert that we're not already at EOF.  Remove
	processing of LF/CR -- there are no operating systems that use
	that.

From-SVN: r275932