PR modula2/111627 defend against ICE

Although PR 111627 can be fixed by renaming testsuite modules it
highlighted that a possible ICE can occur if a malformed
implementation module is actually a program module.  This small
patch defends against this ICE and checks to see whether the module
is a DefImp before testing IsDefinitionForC.

gcc/m2/ChangeLog:

	PR modula2/111627
	PR modula2/112506
	* gm2-compiler/M2Comp.mod (Pass0CheckMod): Test IsDefImp before
	checking IsDefinitionForC.

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