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/parse/context.cpp

@@ -22,7 +22,8 @@
 namespace Carbon::Parse {
 
 Context::Context(Tree* tree, Lex::TokenizedBuffer* tokens,
-                 DiagnosticConsumer* consumer, llvm::raw_ostream* vlog_stream)
+                 Diagnostics::Consumer* consumer,
+                 llvm::raw_ostream* vlog_stream)
     : tree_(tree),
       tokens_(tokens),
       err_tracker_(*consumer),
@@ -283,8 +284,8 @@ auto Context::DiagnoseOperatorFixity(OperatorFixity fixity) -> void {
       CARBON_DIAGNOSTIC(BinaryOperatorRequiresWhitespace, Error,
                         "whitespace missing {0:=-1:before|=0:around|=1:after} "
                         "binary operator",
-                        IntAsSelect);
-      IntAsSelect pos(0);
+                        Diagnostics::IntAsSelect);
+      Diagnostics::IntAsSelect pos(0);
       if (tokens().HasLeadingWhitespace(*position_)) {
         pos.value = 1;
       } else if (tokens().HasTrailingWhitespace(*position_)) {
@@ -302,14 +303,14 @@ auto Context::DiagnoseOperatorFixity(OperatorFixity fixity) -> void {
       CARBON_DIAGNOSTIC(
           UnaryOperatorHasWhitespace, Error,
           "whitespace is not allowed {0:after|before} this unary operator",
-          BoolAsSelect);
+          Diagnostics::BoolAsSelect);
       emitter_.Emit(*position_, UnaryOperatorHasWhitespace, prefix);
     } else if (IsLexicallyValidInfixOperator()) {
       // Pre/postfix operators must not satisfy the infix operator rules.
       CARBON_DIAGNOSTIC(
           UnaryOperatorRequiresWhitespace, Error,
           "whitespace is required {0:before|after} this unary operator",
-          BoolAsSelect);
+          Diagnostics::BoolAsSelect);
       emitter_.Emit(*position_, UnaryOperatorRequiresWhitespace, prefix);
     }
   }