Summary:
`quoteStringForJSON` was operating via indexed access on `StringView`,
performing an `isASCII()` check every iteration and leading to slow
quoting during `JSON.stringify`.

Modify the function to take `UTF16Ref` or `ASCIIRef` directly,
so that it knows the type.

This increases code size slightly while resulting in significant
speedups in `JSON.stringify`.

Reviewed By: neildhar

Differential Revision: D46117526

fbshipit-source-id: e6b849fc8b4e2e748e05d277e36462b8d97ae095