[breaking-change] Remove TypedArena::with_capacity

[nnethercote]

This is a follow-up to #36592.

The function is unused by rustc. Also, it doesn't really follow the
usual meaning of a with_capacity function because the first chunk
allocation is now delayed until the first alloc call.

This change reduces the size of TypedArena by one usize.

@eddyb: we discussed this on IRC. Would you like to review it?

[rust-highfive]

r? @brson

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[eddyb]

@bors r+ Thanks!

[bors]

📌 Commit cf50f5f has been approved by eddyb

[bors]

⌛ Testing commit cf50f5f with merge d6954e8...

[bors]

💔 Test failed - auto-win-gnu-64-opt

[nnethercote]

@eddyb: run-make\output-type-permutations failed. I don't understand what went wrong. Is it the compile warnings about main never being used? I don't know how that relates to my patch. Please help!

[eddyb]

Don't see how it's relevant. @bors retry

[bors]

⌛ Testing commit cf50f5f with merge 7ce16f6...

[bors]

💔 Test failed - auto-mac-64-opt-rustbuild

[nnethercote]

@eddyb: another failure that's not my fault? It's an exception in Python code...

[eddyb]

The bots may be acting up, but it's a weekend so I don't think I have any way of confirming. @bors retry

[bors]

⌛ Testing commit cf50f5f with merge a0843d7...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: eddyb
Pushing a0843d7 to master...