Auto merge of #17705 - huntc:resolve-ra, r=Veykril

feat: Use oldest rustup rust-analyzer when toolchain override is present

Selects a rust-toolchain declared RA based on its date. The earliest (oldest) RA wins and becomes the one that the workspace uses as a whole.

In terms of precedence:

nightly > stable-with-version > stable

With stable-with-version, we invoke the RA with a `--version` arg and attempt to extract a date. Given the same date as a nightly, the nightly RA will win.

Fixes #17663

Author: bors