Make pretty printing for f16 and f128 consistent

Currently the docs show e.g.

    {transmute(0xfffeffffffffffffffffffffffffffff): f128}

for f128 constants. This should fix that to instead use apfloat for
printing, as is done for `f32` and `f64`.

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -7,7 +7,7 @@ use crate::ty::{
     ConstInt, Expr, ParamConst, ScalarInt, Term, TermKind, TypeFoldable, TypeSuperFoldable,
     TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
 };
-use rustc_apfloat::ieee::{Double, Single};
+use rustc_apfloat::ieee::{Double, Half, Quad, Single};
 use rustc_apfloat::Float;
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_data_structures::unord::UnordMap;
@@ -1710,6 +1710,10 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
             ty::Bool if int == ScalarInt::FALSE => p!("false"),
             ty::Bool if int == ScalarInt::TRUE => p!("true"),
             // Float
+            ty::Float(ty::FloatTy::F16) => {
+                let val = Half::try_from(int).unwrap();
+                p!(write("{}{}f16", val, if val.is_finite() { "" } else { "_" }))
+            }
             ty::Float(ty::FloatTy::F32) => {
                 let val = Single::try_from(int).unwrap();
                 p!(write("{}{}f32", val, if val.is_finite() { "" } else { "_" }))
@@ -1718,6 +1722,10 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                 let val = Double::try_from(int).unwrap();
                 p!(write("{}{}f64", val, if val.is_finite() { "" } else { "_" }))
             }
+            ty::Float(ty::FloatTy::F128) => {
+                let val = Quad::try_from(int).unwrap();
+                p!(write("{}{}f128", val, if val.is_finite() { "" } else { "_" }))
+            }
             // Int
             ty::Uint(_) | ty::Int(_) => {
                 let int =