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_ast/src/ast.rs

@@ -1920,22 +1920,28 @@ pub struct FnSig {
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
 #[derive(Encodable, Decodable, HashStable_Generic)]
 pub enum FloatTy {
+    F16,
     F32,
     F64,
+    F128,
 }
 
 impl FloatTy {
     pub fn name_str(self) -> &'static str {
         match self {
+            FloatTy::F16 => "f16",
             FloatTy::F32 => "f32",
             FloatTy::F64 => "f64",
+            FloatTy::F128 => "f128",
         }
     }
 
     pub fn name(self) -> Symbol {
         match self {
+            FloatTy::F16 => sym::f16,
             FloatTy::F32 => sym::f32,
             FloatTy::F64 => sym::f64,
+            FloatTy::F128 => sym::f128,
         }
     }
 }