Remove uses of `_` in defaults

[mark-i-m]

Quoting myself from the referenced thread:

I feel like the internal use of _ as a default name for things is a bit undisciplined, and it's leaking into diagnostics in annoying ways. There is also this: #66679

I feel like internally, we should keep a strong separation between names that we get from the source code being compiled and names being generated by the compiler...

Originally posted by @mark-i-m in #67565 (comment)

cc @Centril @matthewjasper @pnkfelix