Move diagnostics into a namespace (#5173)

What this really does is avoids shadowing names, so that we can
comfortable have things like `Check::DiagnosticEmitter` or
`Check::DiagnosticLoc` without shadowing being a concern.

Note, down this path I'm also thinking about:

- Renaming misc DiagnosticConsumer/DiagnosticEmitter classes, possibly
just to DiagnosticConsumer/DiagnosticEmitter (so
`Check::DiagnosticEmitter` instead of `SemIRLocDiagnosticEmitter`).
- Dropping `Diagnostic` from `Emitter::DiagnosticBuilder`.
- But not for `Check::DiagnosticBuilder`, because `Check::Builder` would
be ambiguous.
- Renaming diagnostics/diagnostic_* to drop "diagnostic".

[Discussion about SemIRLoc ->
DiagnosticLoc](https://discord.com/channels/655572317891461132/655578254970716160/1353771570463768698)
reminded me of this (in particular the older [Check::DiagnosticBuilder
discussion](https://discord.com/channels/655572317891461132/655578254970716160/1344363562608627763)),
but I'd only do that rename if there's matching consensus about a path
forward where we keep SemIRLoc, and in a way that it's only ever used
for diagnostics (the divergence from which is at the root of current
LocId discussion).

I'm trying to keep that separate from a namespace addition for clarity.

toolchain/language_server/context.cpp

@@ -22,7 +22,7 @@ namespace Carbon::LanguageServer {
 // A consumer for turning diagnostics into a `textDocument/publishDiagnostics`
 // notification.
 // https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_publishDiagnostics
-class PublishDiagnosticConsumer : public DiagnosticConsumer {
+class PublishDiagnosticConsumer : public Diagnostics::Consumer {
  public:
   // Initializes params with the target file information.
   explicit PublishDiagnosticConsumer(Context* context,
@@ -31,14 +31,14 @@ class PublishDiagnosticConsumer : public DiagnosticConsumer {
       : context_(context), params_{.uri = uri, .version = version} {}
 
   // Turns a diagnostic into an LSP diagnostic.
-  auto HandleDiagnostic(Diagnostic diagnostic) -> void override {
+  auto HandleDiagnostic(Diagnostics::Diagnostic diagnostic) -> void override {
     const auto& message = diagnostic.messages[0];
     if (message.loc.filename != params_.uri.file()) {
       // `pushDiagnostic` requires diagnostics to be associated with a location
       // in the current file. Suppress diagnostics rooted in other files.
       // TODO: Consider if there's a better way to handle this.
       RawStringOstream stream;
-      StreamDiagnosticConsumer consumer(&stream);
+      Diagnostics::StreamConsumer consumer(&stream);
       consumer.HandleDiagnostic(diagnostic);
 
       CARBON_DIAGNOSTIC(LanguageServerDiagnosticInWrongFile, Warning,
@@ -68,7 +68,7 @@ class PublishDiagnosticConsumer : public DiagnosticConsumer {
  private:
   // Returns the LSP range for a diagnostic. Note that Carbon uses 1-based
   // numbers while LSP uses 0-based.
-  auto GetRange(const DiagnosticLoc& loc) -> clang::clangd::Range {
+  auto GetRange(const Diagnostics::Loc& loc) -> clang::clangd::Range {
     return {.start = {.line = loc.line_number - 1,
                       .character = loc.column_number - 1},
             .end = {.line = loc.line_number,
@@ -76,7 +76,7 @@ class PublishDiagnosticConsumer : public DiagnosticConsumer {
   }
 
   // Converts a diagnostic level to an LSP severity.
-  auto GetSeverity(DiagnosticLevel level) -> int {
+  auto GetSeverity(Diagnostics::Level level) -> int {
     // https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnosticSeverity
     enum class DiagnosticSeverity {
       Error = 1,
@@ -86,9 +86,9 @@ class PublishDiagnosticConsumer : public DiagnosticConsumer {
     };
 
     switch (level) {
-      case DiagnosticLevel::Error:
+      case Diagnostics::Level::Error:
         return static_cast<int>(DiagnosticSeverity::Error);
-      case DiagnosticLevel::Warning:
+      case Diagnostics::Level::Warning:
         return static_cast<int>(DiagnosticSeverity::Warning);
       default:
         CARBON_FATAL("Unexpected diagnostic level: {0}", level);