[Core] Add Twine support for StringAttr and Identifier. NFC.

[Core] Add Twine support for StringAttr and Identifier. NFC.

This is both more efficient and more ergonomic than going
through an std::string, e.g. when using llvm::utostr and
in string concat cases.

Unfortunately we can't just overload ::get().  This causes an
ambiguity because both twine and stringref implicitly convert
from std::string.

Differential Revision: https://reviews.llvm.org/D103754