PR modula2/115957 ICE on procedure local const declaration

PR modula2/115957 ICE on procedure local const declaration

An ICE would occur if a constant was declared using a variable term.
This fix catches variable terms in constant expressions and generates
an unrecoverable error.

gcc/m2/ChangeLog:

	PR modula2/115957
	* gm2-compiler/M2StackAddress.mod (PopAddress): Detect tail=NIL
	and generate an internal error.
	* gm2-compiler/PCBuild.bnf (InConstParameter): New variable.
	(InConstBlock): New variable.
	(ErrorString): Rewrite using MetaErrorStringT0.
	(ErrorArrayAt): Rewrite using MetaErrorStringT0.
	(WarnMissingToken): Use MetaErrorStringT0.
	(CompilationUnit): Set seenError FALSE.
	(init): Initialize InConstParameter and InConstBlock.
	(ConstantDeclaration): Set InConstBlock.
	(ConstSetOrQualidentOrFunction): Call CheckNotVar if not
	InConstParameter and InConstBlock.
	(ConstActualParameters): Set InConstParameter TRUE and restore
	value at the end.
	* gm2-compiler/PCSymBuild.def (CheckNotVar): New procedure.
	Remove all unnecessary export qualified list.
	* gm2-compiler/PCSymBuild.mod (CheckNotVar): New procedure.

gcc/testsuite/ChangeLog:

	PR modula2/115957
	* gm2/errors/fail/badconst.mod: New test.
	* gm2/pim/fail/tinyadr.mod: New test.

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