Remove lookup logic that attempts to locate a dialect for a type,
just use StrSQLTypeCompiler.
Cleaned up the internal ``str()`` for datatypes so that all types produce a
string representation without any dialect present, including that it works
for third-party dialect types without that dialect being present. The
string representation defaults to being the UPPERCASE name of that type
with nothing else.
Fixes: #4262
Change-Id: I02149e8a1ba1e7336149e962939b07ae0df83c6b
Mike Bayer
0721a6bede