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/source/source_buffer_test.cpp

@@ -17,8 +17,8 @@ static constexpr llvm::StringLiteral TestFileName = "test.carbon";
 
 TEST(SourceBufferTest, MissingFile) {
   llvm::vfs::InMemoryFileSystem fs;
-  auto buffer =
-      SourceBuffer::MakeFromFile(fs, TestFileName, ConsoleDiagnosticConsumer());
+  auto buffer = SourceBuffer::MakeFromFile(fs, TestFileName,
+                                           Diagnostics::ConsoleConsumer());
   EXPECT_FALSE(buffer);
 }
 
@@ -27,8 +27,8 @@ TEST(SourceBufferTest, SimpleFile) {
   CARBON_CHECK(fs.addFile(TestFileName, /*ModificationTime=*/0,
                           llvm::MemoryBuffer::getMemBuffer("Hello World")));
 
-  auto buffer =
-      SourceBuffer::MakeFromFile(fs, TestFileName, ConsoleDiagnosticConsumer());
+  auto buffer = SourceBuffer::MakeFromFile(fs, TestFileName,
+                                           Diagnostics::ConsoleConsumer());
   ASSERT_TRUE(buffer);
 
   EXPECT_EQ(TestFileName, buffer->filename());
@@ -44,8 +44,8 @@ TEST(SourceBufferTest, NoNull) {
                                        /*BufferName=*/"",
                                        /*RequiresNullTerminator=*/false)));
 
-  auto buffer =
-      SourceBuffer::MakeFromFile(fs, TestFileName, ConsoleDiagnosticConsumer());
+  auto buffer = SourceBuffer::MakeFromFile(fs, TestFileName,
+                                           Diagnostics::ConsoleConsumer());
   ASSERT_TRUE(buffer);
 
   EXPECT_EQ(TestFileName, buffer->filename());
@@ -57,8 +57,8 @@ TEST(SourceBufferTest, EmptyFile) {
   CARBON_CHECK(fs.addFile(TestFileName, /*ModificationTime=*/0,
                           llvm::MemoryBuffer::getMemBuffer("")));
 
-  auto buffer =
-      SourceBuffer::MakeFromFile(fs, TestFileName, ConsoleDiagnosticConsumer());
+  auto buffer = SourceBuffer::MakeFromFile(fs, TestFileName,
+                                           Diagnostics::ConsoleConsumer());
   ASSERT_TRUE(buffer);
 
   EXPECT_EQ(TestFileName, buffer->filename());