[mlir][Parser] Make parse{Attribute,Type} null-terminate input

`parseAttribute` and `parseType` require null-terminated strings as
input, but this isn't great considering the argument type is
`StringRef`. This changes them to copy to a null-terminated buffer by
default, with a `isKnownNullTerminated` flag added to disable the
copying.

closes #58964

Reviewed By: rriddle, kuhar, lattner

Differential Revision: https://reviews.llvm.org/D145182