Rewrite the untranslatable_diagnostic lint.

Currently it only checks calls to functions marked with
`#[rustc_lint_diagnostics]`. This commit changes it to check calls to
any function with an `impl Into<{D,Subd}iagMessage>` parameter. This
greatly improves its coverage and doesn't rely on people remembering to
add `#[rustc_lint_diagnostics]`.

The commit also adds `#[allow(rustc::untranslatable_diagnostic)`]
attributes to places that need it that are caught by the improved lint.
These places that might be easy to convert to translatable diagnostics.

Finally, it also:
- Expands and corrects some comments.
- Does some minor formatting improvements.
- Adds missing `DecorateLint` cases to
  `tests/ui-fulldeps/internal-lints/diagnostics.rs`.

compiler/rustc_session/src/config.rs

@@ -1,6 +1,8 @@
 //! Contains infrastructure for configuring the compiler, including parsing
 //! command-line options.
 
+#![allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
+
 pub use crate::options::*;
 
 use crate::errors::FileWriteFail;
@@ -2468,9 +2470,7 @@ pub fn parse_externs(
             ));
             let adjusted_name = name.replace('-', "_");
             if is_ascii_ident(&adjusted_name) {
-                // FIXME: make this translatable
-                #[allow(rustc::diagnostic_outside_of_impl)]
-                #[allow(rustc::untranslatable_diagnostic)]
+                #[allow(rustc::diagnostic_outside_of_impl)] // FIXME
                 error.help(format!(
                     "consider replacing the dashes with underscores: `{adjusted_name}`"
                 ));

compiler/rustc_session/src/options.rs

@@ -320,6 +320,7 @@ macro_rules! redirect_field {
 type OptionSetter<O> = fn(&mut O, v: Option<&str>) -> bool;
 type OptionDescrs<O> = &'static [(&'static str, OptionSetter<O>, &'static str, &'static str)];
 
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 fn build_options<O: Default>(
     early_dcx: &EarlyDiagCtxt,
     matches: &getopts::Matches,

compiler/rustc_session/src/search_paths.rs

@@ -61,6 +61,7 @@ impl SearchPath {
             (PathKind::All, path)
         };
         if path.is_empty() {
+            #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
             early_dcx.early_fatal("empty search path given via `-L`");
         }
 

compiler/rustc_session/src/session.rs

@@ -1022,6 +1022,7 @@ fn default_emitter(
 
 // JUSTIFICATION: literally session construction
 #[allow(rustc::bad_opt_access)]
+#[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
 pub fn build_session(
     early_dcx: EarlyDiagCtxt,
     sopts: config::Options,