Rollup merge of #121841 - tgross35:f16-f128-step2-intrinsics, r=compiler-errors

`f16` and `f128` step 2: intrinsics

Continuation of https://github.com/rust-lang/rust/pull/121728, another portion of https://github.com/rust-lang/rust/pull/114607.

This PR adds `f16` and `f128` intrinsics, and hooks them up to both HIR and LLVM. This is all still unexposed to the frontend, which will probably be the next step. Also update itanium mangling per `@rcvalle's` in https://github.com/rust-lang/rust/pull/121728/files#r1506570300, and fix a typo from step 1.

Once these types are usable in code, I will add the codegen tests from #114607 (codegen is passing on that branch)

This does add more `unimplemented!`s to Clippy, but I still don't think we can do better until library support is added.

r? `@compiler-errors`
cc `@Nilstrieb`
`@rustbot` label +T-compiler +F-f16_and_f128

compiler/rustc_middle/src/ty/mod.rs

@@ -1991,8 +1991,10 @@ pub fn uint_ty(uty: ast::UintTy) -> UintTy {
 
 pub fn float_ty(fty: ast::FloatTy) -> FloatTy {
     match fty {
+        ast::FloatTy::F16 => FloatTy::F16,
         ast::FloatTy::F32 => FloatTy::F32,
         ast::FloatTy::F64 => FloatTy::F64,
+        ast::FloatTy::F128 => FloatTy::F128,
     }
 }