[mlir][AsmParser] Improve parse{Attribute,Type} error handling

Currently these functions report errors directly to stderr, this updates
them to use diagnostics instead. This also makes partially-consumed
strings an error if the `numRead` parameter isn't provided (the
docstrings already claimed this happened, but it didn't.)

While here I also tried to reduce the number of overloads by switching
to using default parameters.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D144804