Optionally emit errors from IntegerType factory functions.

Similarly to other types, introduce "get" and "getChecked" static member
functions for IntegerType.  The latter emits errors to the error handler
registered with the MLIR context and returns a null type for the caller to
handle errors gracefully.  This deduplicates type consistency checks between
the parser and the builder.  Update the parser to call IntegerType::getChecked
for error reporting instead of the builder that would simply assert.

This CL completes the type system error emission refactoring: the parser now
only emits syntax-related errors for types while type factory systems may emit
type consistency errors.

PiperOrigin-RevId: 221165207