PR modula2/121354: ICE when attempting to fold HIGH from an unbounded array in a nested procedure

The bug fix re-implements gcc/m2/gm2-compiler/M2GenGCC.mod:FoldHigh
to ignore any attempt to constant fold HIGH if it has an unbounded
array operand.

gcc/m2/ChangeLog:

	PR modula2/121354
	* gm2-compiler/M2GenGCC.mod (FoldHigh): Rewrite.
	(IsUnboundedArray): New procedure function.

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