[flang] Fix resolution of function prefix type

When a function-stmt has a type in the prefix (`type(t) function f()`),
the type cannot be resolve until after processing the USE and IMPLICIT
statements. So save the parse-tree of the type and process it at the
end of the specification section.

Add `ProcessTypeSpec()` to handle the process of setting up to walk
a type spec, walking it, restoring the state, and returning the type
spec. We do this several other places too.

Original-commit: flang-compiler/f18@bcde294d0e62c6ef8506392f66816814ae976dff
Reviewed-on: https://github.com/flang-compiler/f18/pull/305
Tree-same-pre-rewrite: false