Put constructed name for gobal constant segment in its global variable.

The global constant segment comes from the front end with no name,
in the declare_segment operator.

M3CG_LLVM was already concocting a name for it ("M_Const"), but
not storing it in the variable for the segment, leading to a NIL
segfault later, when handling the end_init operator, in DebugGlobals,
while calling M3DIB.DIBcreateGlobalVariable.

m3-sys/llvm3.6.1/src/M3CG_LLVM.m3

@@ -1437,6 +1437,7 @@ PROCEDURE declare_segment (self: U;  n: Name;  m3t: TypeUID; is_const: BOOLEAN):
     IF is_const THEN
       (* the name will be nil so create a const name *)
       segName := "M_Const";
+      v.name := M3ID.Add(segName); 
       (* keep the value for globals debugging *)
       self.segConst := v;
     ELSE