Change `_read` to `yielding borrow`

[grynspan]

Change our uses of the experimental _read to yielding borrow. The latter is ABI-compatible with the former per SE-0474

[grynspan]

Not planning to do this for 6.2 as it's harmless as-is, we want to support 6.1 toolchains until 6.2 is released, and the compiler implementation hasn't fully settled yet. Will aim for the next release (presumably 6.3).

[stmontgomery]

Tracked by rdar://154670924