Add an offset to StackSlotData.

The offset is relative to the stack pointer in the calling function, so
it excludes the return address pushed by the call instruction itself on
Intel ISAs.

Change the ArgumentLoc::Stack offset to an i32, so it matches the stack
slot offsets.

filetests/parser/tiny.cton

@@ -98,7 +98,7 @@ ebb0(v90: i32, v91: f32):
 function %stack() {
     ss10 = spill_slot 8
     ss2 = local 4
-    ss3 = incoming_arg 4
+    ss3 = incoming_arg 4, offset 8
     ss4 = outgoing_arg 4
 
 ebb0:
@@ -110,7 +110,7 @@ ebb0:
 ; sameln: function %stack() {
 ; nextln:    $ss10 = spill_slot 8
 ; nextln:    $ss2 = local 4
-; nextln:    $ss3 = incoming_arg 4
+; nextln:    $ss3 = incoming_arg 4, offset 8
 ; nextln:    $ss4 = outgoing_arg 4
 
 ; check: ebb0: