Make all thir types implement clone

[xldenis]

This PR adds Clone impl to all of the Thir<'tcx> types.

I would like to be able to clone a Thir body so that I can make a copy in my rustc driver without breaking further compilation. Without this my driver is forced to run in the after_expansion callback and thus doesn't benefit from running all the safety checks that rustc usually does, instead i need to do them all myself.

[rust-highfive]

r? @petrochenkov

(rust-highfive has picked a reviewer for you, use r? to override)

[petrochenkov]

I don't know what invariants THIR is supposed to have and why it's not Clone now.
r? @LeSeulArtichaut (as suggested by git blame)

If the Clone impls are added, then you'll need to add a comment to struct Thir explaining why they are necessary, otherwise someone will remove them in some next cleanup.

[petrochenkov]

r? @b-naber

[petrochenkov]

Ok.
r? rust-lang/compiler

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit eed91ee has been approved by oli-obk

[bors]

⌛ Testing commit eed91ee with merge 41ef767...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 41ef767 to master...

[rust-timer]

Finished benchmarking commit (41ef767): comparison url.

Summary:

• Primary benchmarks: 😿 relevant regressions found
• Secondary benchmarks: no relevant changes found

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	2	0	0	0	2
mean2	0.5%	N/A	N/A	N/A	0.5%
max	0.6%	N/A	N/A	N/A	0.6%

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. number of relevant changes ↩

2. the arithmetic mean of the percent change ↩