PR modula2/117120: case ch with a nul char constant causes ICE

PR modula2/117120: case ch with a nul char constant causes ICE

This patch fixes the ICE caused when a case clause contains
a character constant ''.  The fix was to walk the caselist and
convert any 0 length string into a char constant of value 0.

gcc/m2/ChangeLog:

	PR modula2/117120
	* gm2-compiler/M2CaseList.mod (CaseBoundsResolved): Rewrite.
	(ConvertNulStr2NulChar): New procedure function.
	(NulStr2NulChar): Ditto.
	(GetCaseExpression): Ditto.
	(OverlappingCaseBound): Rewrite.
	* gm2-compiler/M2GCCDeclare.mod (CheckResolveSubrange): Allow
	'' to be used as the subrange low limit.
	* gm2-compiler/M2GenGCC.mod (FoldConvert): Rewrite.
	(PopKindTree): Ditto.
	(BuildHighFromString): Reformat.
	* gm2-compiler/SymbolTable.mod (PushConstString): Add test for
	length 0 and PushChar (nul).

gcc/testsuite/ChangeLog:

	PR modula2/117120
	* gm2/pim/pass/forloopnulchar.mod: New test.
	* gm2/pim/pass/nulcharcase.mod: New test.
	* gm2/pim/pass/nulcharvar.mod: New test.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>