Optimize the code produced by derive(Debug).

This commit adds new methods that combine sequences of existing
formatting methods.
- `Formatter::debug_{tuple,struct}_field[12345]_finish`, equivalent to a
  `Formatter::debug_{tuple,struct}` + N x `Debug{Tuple,Struct}::field` +
  `Debug{Tuple,Struct}::finish` call sequence.
- `Formatter::debug_{tuple,struct}_fields_finish` is similar, but can
  handle any number of fields by using arrays.

These new methods are all marked as `doc(hidden)` and unstable. They are
intended for the compiler's own use.

Special-casing up to 5 fields gives significantly better performance
results than always using arrays (as was tried in #95637).

The commit also changes the `Debug` deriving code to use these new methods. For
example, where the old `Debug` code for a struct with two fields would be like
this:
```
fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
    match *self {
	Self {
	    f1: ref __self_0_0,
	    f2: ref __self_0_1,
	} => {
	    let debug_trait_builder = &mut ::core::fmt::Formatter::debug_struct(f, "S2");
	    let _ = ::core::fmt::DebugStruct::field(debug_trait_builder, "f1", &&(*__self_0_0));
	    let _ = ::core::fmt::DebugStruct::field(debug_trait_builder, "f2", &&(*__self_0_1));
	    ::core::fmt::DebugStruct::finish(debug_trait_builder)
	}
    }
}
```
the new code is like this:
```
fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
    match *self {
	Self {
	    f1: ref __self_0_0,
	    f2: ref __self_0_1,
	} => ::core::fmt::Formatter::debug_struct_field2_finish(
	    f,
	    "S2",
	    "f1",
	    &&(*__self_0_0),
	    "f2",
	    &&(*__self_0_1),
	),
    }
}
```
This shrinks the code produced for `Debug` instances
considerably, reducing compile times and binary sizes.

Co-authored-by: Scott McMurray <scottmcm@users.noreply.github.com>

compiler/rustc_span/src/symbol.rs

@@ -567,8 +567,10 @@ symbols! {
         debug_assert_ne_macro,
         debug_assertions,
         debug_struct,
+        debug_struct_fields_finish,
         debug_trait_builder,
         debug_tuple,
+        debug_tuple_fields_finish,
         debugger_visualizer,
         decl_macro,
         declare_lint_pass,