Add `f16` and `f128` support (add to `FloatTy`)

[tgross35]

FloatTy in rustc has f16 and f128 now. It would probably be pretty easy to update RA's FloatTy to match and propagate that out.

[tgross35]

Probably pretty easy

@rustbot label +E-easy +A-hir

[rustbot]

Error: Label E-easy can only be set by Rust team members

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

[Veykril]

That will require changes to chalk

[beetrees]

Now that #17572 has been merged, all that's left for this issue is to:

• Add support for f16/f128 intrinsics.
• Replace rustc_apfloat with the types themselves once they've been stabilised.

Most of the former will probably require the later since rustc_apfloat only supports basic arithmetic operations.