PR modula2/119504: ICE when attempting to access an element of a constant string

PR modula2/119504: ICE when attempting to access an element of a constant string

This patch prevents an ICE and generates an error if an array access to a
constant string is attempted.  The patch also allows HIGH ("string").

gcc/m2/ChangeLog:

	PR modula2/119504
	* gm2-compiler/M2Quads.mod (BuildHighFunction): Defend against
	Type = NulSym and fall into BuildConstHighFromSym.
	(BuildDesignatorArray): Rewrite to detect an array access to
	a constant string.
	(BuildDesignatorArrayStaticDynamic): New procedure.

gcc/testsuite/ChangeLog:

	PR modula2/119504
	* gm2/iso/fail/conststrarray2.mod: New test.
	* gm2/iso/run/pass/constarray2.mod: New test.
	* gm2/pim/pass/hexstring.mod: New test.

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