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/lex/tokenized_buffer.cpp

@@ -378,7 +378,7 @@ auto TokenizedBuffer::CollectMemUsage(MemUsage& mem_usage,
 }
 
 auto TokenizedBuffer::SourcePointerToDiagnosticLoc(const char* loc) const
-    -> ConvertedDiagnosticLoc {
+    -> Diagnostics::ConvertedLoc {
   CARBON_CHECK(StringRefContainsPointer(source_->text(), loc),
                "location not within buffer");
   int32_t offset = loc - source_->text().begin();
@@ -416,7 +416,7 @@ auto TokenizedBuffer::SourcePointerToDiagnosticLoc(const char* loc) const
 }
 
 auto TokenizedBuffer::TokenToDiagnosticLoc(TokenIndex token) const
-    -> ConvertedDiagnosticLoc {
+    -> Diagnostics::ConvertedLoc {
   // Map the token location into a position within the source buffer.
   const char* token_start =
       source_->text().begin() + GetTokenInfo(token).byte_offset();