Auto merge of #155223 - teor2345:fndef-refactor, r=mati865

Refactor FnDecl and FnSig non-type fields into a new wrapper type





#### Why this Refactor?

This PR is part of an initial cleanup for the [arg splat experiment](rust-lang/rust#153629), but it's a useful refactor by itself.

It refactors the non-type fields of `FnDecl`, `FnSig`, and `FnHeader` into a new packed wrapper types, based on this comment in the `splat` experiment PR:
rust-lang/rust#153697 (comment)

It also refactors some common `FnSig` creation settings into their own methods. I did this instead of creating a struct with defaults.

#### Relationship to `splat` Experiment

I don't think we can use functional struct updates (`..default()`) to create `FnDecl` and `FnSig`, because we need the bit-packing for the `splat` experiment.

Bit-packing will avoid breaking "type is small" assertions for commonly used types when `splat` is added.
This PR packs these types:
- ExternAbi: enum + `unwind` variants (38) -> 6 bits
- ImplicitSelfKind: enum variants (5) -> 3 bits
- lifetime_elision_allowed, safety, c_variadic: bool -> 1 bit

#### Minor Changes

Fixes some typos, and applies rustfmt to clippy files that got skipped somehow.

Author: bors

crates/stdarch-gen-arm/src/intrinsic.rs

@@ -550,7 +550,7 @@ impl LLVMLink {
 
     /// Alters all the unsigned types from the signature. This is required where
     /// a signed and unsigned variant require the same binding to an exposed
-    /// LLVM instrinsic.
+    /// LLVM intrinsic.
     pub fn sanitise_uints(&mut self) {
         let transform = |tk: &mut TypeKind| {
             if let Some(BaseType::Sized(BaseTypeKind::UInt, size)) = tk.base_type() {
@@ -1138,7 +1138,7 @@ impl Intrinsic {
             } else {
                 /* If we do not need to reorder anything then immediately add
                  * the expressions from the big_endian_expressions and
-                 * concatinate the compose vector */
+                 * concatenate the compose vector */
                 variant.big_endian_compose.extend(big_endian_expressions);
                 variant
                     .big_endian_compose
@@ -1156,11 +1156,11 @@ impl Intrinsic {
 
             /* If we do not create a shuffle call we do not need modify the
              * return value and append to the big endian ast array. A bit confusing
-             * as in code we are making the final call before caputuring the return
+             * as in code we are making the final call before capturing the return
              * value of the intrinsic that has been called.*/
             let ret_val_name = "ret_val".to_string();
             if let Some(simd_shuffle_call) = create_shuffle_call(&ret_val_name, return_type) {
-                /* There is a possibility that the funcion arguments did not
+                /* There is a possibility that the function arguments did not
                  * require big endian treatment, thus we need to now add the
                  * original function body before appending the return value.*/
                 if variant.big_endian_compose.is_empty() {
@@ -1694,8 +1694,8 @@ enum Endianness {
     NA,
 }
 
-/// Based on the endianess will create the appropriate intrinsic, or simply
-/// create the desired intrinsic without any endianess
+/// Based on the endianness will create the appropriate intrinsic, or simply
+/// create the desired intrinsic without any endianness
 fn create_tokens(intrinsic: &Intrinsic, endianness: Endianness, tokens: &mut TokenStream) {
     let signature = &intrinsic.signature;
     let fn_name = signature.fn_name().to_string();