[MLIR][ODS] default-valued strings should be in quotes

`DefaultValuedAttr<StrAttr, "">` and `ConstantAttr<StrAttr, "">`
result in bugs in which TableGen will not recognize that the attribute
has a default value, because `""` is an empty TableGen string.

Strings no longer have special treatment. Instead, string values must be
wrapped in quotes: "\"foo\"". Two helpers, `DefaultValuedStrAttr` and
`ConstantStrAttr` have been added to keep code clean.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D111855