verilog: fix multiple AST_PREFIX scope resolution issues

- Root AST_PREFIX nodes are now subject to genblk expansion to allow
  them to refer to a locally-visible generate block
- Part selects on AST_PREFIX member leafs can now refer to generate
  block items (previously would not resolve and raise an error)
- Add source location information to AST_PREFIX nodes

tests/verilog/prefix.ys

@@ -0,0 +1,5 @@
+read_verilog -sv prefix.sv
+hierarchy
+proc
+select -module top
+sat -verify -seq 1 -prove-asserts -show-all