d: Fix ICE in when formating a string with '%' or '`' characters (PR98457)

The percentage character was being confused for a format specifier in
pp_format(), whilst the backtick character was confused for the
beginning of a quoted string in expand_d_format().

Both are now properly escaped to avoid the ICE.

gcc/d/ChangeLog:

	PR d/98457
	* d-diagnostic.cc (expand_d_format): Handle escaped backticks.
	(escape_d_format): New funtion.
	(verror): Call escape_d_format on prefixing strings.
	(vdeprecation): Likewise.

gcc/testsuite/ChangeLog:

	PR d/98457
	* gdc.dg/pr98457.d: New test.