Auto merge of #113843 - wesleywiser:replace_rustc_apfloat, r=pnkfelix

Replace in-tree `rustc_apfloat` with the new version of the crate

Replace the in-tree version of `rustc_apfloat` with the new version of the crate which has been correctly licensed. The new crate incorporates upstream changes from LLVM since the original port was done including many correctness fixes and has been extensively fuzz tested to validate correctness.

Fixes #100233
Fixes #102403
Fixes #113407
Fixes #113409
Fixes #55993
Fixes #93224
Closes #93225
Closes #109573

Author: bors

src/shims/foreign_items.rs

@@ -914,16 +914,6 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let x = this.read_scalar(x)?.to_f64()?;
                 let exp = this.read_scalar(exp)?.to_i32()?;
 
-                // Saturating cast to i16. Even those are outside the valid exponent range so
-                // `scalbn` below will do its over/underflow handling.
-                let exp = if exp > i32::from(i16::MAX) {
-                    i16::MAX
-                } else if exp < i32::from(i16::MIN) {
-                    i16::MIN
-                } else {
-                    exp.try_into().unwrap()
-                };
-
                 let res = x.scalbn(exp);
                 this.write_scalar(Scalar::from_f64(res), dest)?;
             }