[mlir] Add a DialectAsmParser::getChecked method

This function simplifies calling the getChecked methods on Attributes and Types from within the parser, and removes any need to use `getEncodedSourceLocation` for these methods (by using an SMLoc instead). This is much more efficient than using an mlir::Location, as the encoding process to produce an mlir::Location is inefficient and undesirable for parsing (locations used during parsing should not persist afterwards unless otherwise necessary).

Differential Revision: https://reviews.llvm.org/D97900