[mlir] Add a new AttrTypeReplacer class to simplify sub element replacements

[mlir] Add a new AttrTypeReplacer class to simplify sub element replacements

We currently only have the SubElement interface API for attribute/type
replacement, but this suffers from several issues; namely that it doesn't
allow caching across multiple replacements (very common), and also
creates a somewhat awkward/limited API. The new AttrTypeReplacer class
allows for registering replacements using a much cleaner API, similarly to
the TypeConverter class, removes a lot of manual interaction with the
sub element interfaces, and also better enables large scale replacements.

Differential Revision: https://reviews.llvm.org/D137764