Refactor diagnostic_emitter.h (#4871)

I'm planning on eliminating DiagnosticConverter. As part of this,
collapse it into diagnostic_emitter.h, and refactoring the header a
little so that it's more readable when making changes.

Author: jonmeow

toolchain/check/sem_ir_diagnostic_converter.h

@@ -7,7 +7,7 @@
 
 #include "llvm/ADT/ArrayRef.h"
 #include "toolchain/check/diagnostic_helpers.h"
-#include "toolchain/diagnostics/diagnostic_converter.h"
+#include "toolchain/diagnostics/diagnostic_emitter.h"
 #include "toolchain/lex/token_index.h"
 #include "toolchain/parse/tree_and_subtrees.h"
 #include "toolchain/sem_ir/file.h"

toolchain/diagnostics/BUILD

@@ -21,7 +21,6 @@ cc_library(
     hdrs = [
         "diagnostic.h",
         "diagnostic_consumer.h",
-        "diagnostic_converter.h",
         "diagnostic_emitter.h",
     ],
     deps = [