Auto merge of #82495 - LeSeulArtichaut:eager-thir, r=oli-obk

Auto merge of #82495 - LeSeulArtichaut:eager-thir, r=oli-obk

Eagerly construct bodies of THIR

With this PR:
 - the THIR is no longer constructed lazily, but is entirely built before being passed to the MIR Builder
 - the THIR is now allocated in arenas instead of `Box`es

However, this PR doesn't make any changes to the way patterns are constructed: they are still boxed, and exhaustiveness checking is unchanged.

Implements MCP rust-lang/compiler-team#409.
Closes rust-lang/project-thir-unsafeck#1.
r? `@ghost` cc `@nikomatsakis` `@oli-obk`