Support most consts in custom mir

[JakobDegen]

First commit is #104975 .

The second commit adds support for most kinds of consts to custom Mir. I lightly refactored the analogous code from regular mir building so that it can be reused.

The only thing that is not yet supported is statics. They're slightly more complicated to deal with because they're actually pointers, so we don't bother in this PR.

r? @oli-obk

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[JakobDegen]

Closing in favor of a single PR