verilog: rebuild user_type_stack from globals before parsing file

verilog: rebuild user_type_stack from globals before parsing file

This was actually a ticking UB bomb: after running the parser, the type
maps contain pointers to children of the current AST, which is
recursively deleted after the pass has executed. This leaves the
pointers in user_type_stack dangling, which just happened to never be a
problem due to another bug that causes typedefs from higher-level type
maps to never be considered.

Rebuilding the type stack from the design's globals ensures the AstNode
pointers are valid.