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/structured_errors/missing_cast_for_variadic_arg.rs

@@ -1,5 +1,5 @@
 use crate::{errors, structured_errors::StructuredDiagnostic};
-use rustc_errors::DiagnosticBuilder;
+use rustc_errors::{codes::*, DiagnosticBuilder, ErrCode};
 use rustc_middle::ty::{Ty, TypeVisitableExt};
 use rustc_session::Session;
 use rustc_span::Span;
@@ -16,8 +16,8 @@ impl<'tcx> StructuredDiagnostic<'tcx> for MissingCastForVariadicArg<'tcx, '_> {
         self.sess
     }
 
-    fn code(&self) -> String {
-        rustc_errors::error_code!(E0617)
+    fn code(&self) -> ErrCode {
+        E0617
     }
 
     fn diagnostic_common(&self) -> DiagnosticBuilder<'tcx> {

compiler/rustc_hir_analysis/src/structured_errors/sized_unsized_cast.rs

@@ -1,5 +1,5 @@
 use crate::{errors, structured_errors::StructuredDiagnostic};
-use rustc_errors::DiagnosticBuilder;
+use rustc_errors::{codes::*, DiagnosticBuilder, ErrCode};
 use rustc_middle::ty::{Ty, TypeVisitableExt};
 use rustc_session::Session;
 use rustc_span::Span;
@@ -16,8 +16,8 @@ impl<'tcx> StructuredDiagnostic<'tcx> for SizedUnsizedCast<'tcx> {
         self.sess
     }
 
-    fn code(&self) -> String {
-        rustc_errors::error_code!(E0607)
+    fn code(&self) -> ErrCode {
+        E0607
     }
 
     fn diagnostic_common(&self) -> DiagnosticBuilder<'tcx> {

compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs

@@ -1,5 +1,7 @@
 use crate::structured_errors::StructuredDiagnostic;
-use rustc_errors::{pluralize, Applicability, Diagnostic, DiagnosticBuilder, MultiSpan};
+use rustc_errors::{
+    codes::*, pluralize, Applicability, Diagnostic, DiagnosticBuilder, ErrCode, MultiSpan,
+};
 use rustc_hir as hir;
 use rustc_middle::ty::{self as ty, AssocItems, AssocKind, TyCtxt};
 use rustc_session::Session;
@@ -1105,8 +1107,8 @@ impl<'tcx> StructuredDiagnostic<'tcx> for WrongNumberOfGenericArgs<'_, 'tcx> {
         self.tcx.sess
     }
 
-    fn code(&self) -> String {
-        rustc_errors::error_code!(E0107)
+    fn code(&self) -> ErrCode {
+        E0107
     }
 
     fn diagnostic_common(&self) -> DiagnosticBuilder<'tcx> {