Parser: treat implicit top-level module as an SSA name scope

Now that modules are also operations, nothing prevents one from defining SSA
values in the module.  Doing so in an implicit top-level module, i.e. outside
of a `module` operation, was leading to a crash because the implicit module was
not associated with an SSA name scope.  Create a name scope before parsing the
top-level module to fix this.

PiperOrigin-RevId: 262366891