Auto merge of #34956 - nikomatsakis:incr-comp-o-files, r=mw

Auto merge of #34956 - nikomatsakis:incr-comp-o-files, r=mw

Enable reuse of `.o` files if nothing has changed

This PR completes a first "spike" for incremental compilation by enabling us to reuse `.o` files when nothing has changed. When in incr. mode, we will save `.o` files into the temporary directory, then copy them back out again if they are still valid. The code is still a bit rough but it does seem to work. =)

r? @michaelwoerister

Fixes #34036
Fixes #34037
Fixes #34038