[MLIR] Add `KeywordOrString` handling to AsmParser

This adds a new parser and printer for text which may be a keyword or a
string. When printing, it will attempt to print the text as a keyword,
but if it has any special or non-printable characters, it will be
printed as an escaped string.  When parsing, it will parse either a
valid keyword or a potentially escaped string. The printer allows for an
empty string, in which case it prints `""`.

This new function is used for printing the name in NamedAttributes, and
for printing the symbol name after the `@`. In CIRCT we are using this
to print module port names, which are conceptually similar to named
function arguments.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D111683