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_builtin_macros/src/errors.rs

@@ -1,6 +1,6 @@
 use rustc_errors::{
-    AddToDiagnostic, DiagCtxt, DiagnosticBuilder, EmissionGuarantee, IntoDiagnostic, Level,
-    MultiSpan, SingleLabelManySpans,
+    codes::*, AddToDiagnostic, DiagCtxt, DiagnosticBuilder, EmissionGuarantee, IntoDiagnostic,
+    Level, MultiSpan, SingleLabelManySpans,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{symbol::Ident, Span, Symbol};
@@ -269,7 +269,7 @@ pub(crate) struct ConcatIdentsIdentArgs {
 }
 
 #[derive(Diagnostic)]
-#[diag(builtin_macros_bad_derive_target, code = "E0774")]
+#[diag(builtin_macros_bad_derive_target, code = E0774)]
 pub(crate) struct BadDeriveTarget {
     #[primary_span]
     #[label]
@@ -283,7 +283,7 @@ pub(crate) struct BadDeriveTarget {
 pub(crate) struct TestsNotSupport {}
 
 #[derive(Diagnostic)]
-#[diag(builtin_macros_unexpected_lit, code = "E0777")]
+#[diag(builtin_macros_unexpected_lit, code = E0777)]
 pub(crate) struct BadDeriveLit {
     #[primary_span]
     #[label]