[mlir][OpAsmParser] Add parseString method

Basically every kind of parseOptional* method in DialectAsmParser has a corresponding parse* method which will emit an error if the requested token has not been found. An odd one out of this rule is parseOptionalString which does not have a corresponding parseString method.

This patch adds that method and implements it in basically the same fashion as parseKeyword, by first going through parseOptionalString and emitting an error on failure.

Differential Revision: https://reviews.llvm.org/D105406