Stop using String for error codes.

Error codes are integers, but `String` is used everywhere to represent
them. Gross!

This commit introduces `ErrCode`, an integral newtype for error codes,
replacing `String`. It also introduces a constant for every error code,
e.g. `E0123`, and removes the `error_code!` macro. The constants are
imported wherever used with `use rustc_errors::codes::*`.

With the old code, we have three different ways to specify an error code
at a use point:
```
error_code!(E0123)  // macro call

struct_span_code_err!(dcx, span, E0123, "msg");  // bare ident arg to macro call

\#[diag(name, code = "E0123")]  // string
struct Diag;
```

With the new code, they all use the `E0123` constant.
```
E0123  // constant

struct_span_code_err!(dcx, span, E0123, "msg");  // constant

\#[diag(name, code = E0123)]  // constant
struct Diag;
```

The commit also changes the structure of the error code definitions:
- `rustc_error_codes` now just defines a higher-order macro listing the
  used error codes and nothing else.
- Because that's now the only thing in the `rustc_error_codes` crate, I
  moved it into the `lib.rs` file and removed the `error_codes.rs` file.
- `rustc_errors` uses that macro to define everything, e.g. the error
  code constants and the `DIAGNOSTIC_TABLES`. This is in its new
  `codes.rs` file.

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -18,8 +18,8 @@ use crate::require_c_abi_if_c_variadic;
 use rustc_ast::TraitObjectSyntax;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_errors::{
-    error_code, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder,
-    ErrorGuaranteed, FatalError, MultiSpan,
+    codes::*, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
+    FatalError, MultiSpan,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
@@ -1072,9 +1072,9 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 if let Some(binding) = binding
                     && let ConvertedBindingKind::Equality(_) = binding.kind
                 {
-                    error_code!(E0222)
+                    E0222
                 } else {
-                    error_code!(E0221)
+                    E0221
                 },
             );
 
@@ -1630,7 +1630,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             let reported = tcx
                 .dcx()
                 .struct_span_err(span, msg)
-                .with_code(rustc_errors::error_code!(E0624))
+                .with_code(E0624)
                 .with_span_label(span, format!("private {kind}"))
                 .with_span_label(def_span, format!("{kind} defined here"))
                 .emit();