Fix FIXME's/TODOs:

 - Enhance memref type to allow omission of mappings and address
   spaces (implying a default mapping).
 - Fix printing of function types to properly recurse with printType
   so mappings are printed by name.
 - Simplify parsing of AffineMaps a bit now that we have
   isSymbolicOrConstant()

PiperOrigin-RevId: 206039755