Drop "diagnostic" from some filenames in the "diagnostics" folder (#6686)

Mainly because "sorting_diagnostic_consumer" is legacy, since
`SortingDiagnosticConsumer` became `SortingConsumer`. Also better
reflecting contents of these files.

Where I'm not renaming, I'm less positive about dropping "diagnostics"
from "file_diagnostics" and "null_diagnostics" (which contain both a
consumer and emitter, and "null.h" seems like poor naming), so not doing
that here. Also "diagnostic.h" contains `struct Diagnostic`, so is a
decent fit.

Assisted-by: Google Antigravity with Gemini 3 Flash

toolchain/codegen/BUILD

@@ -17,7 +17,7 @@ cc_library(
     hdrs = ["codegen.h"],
     deps = [
         "//common:check",
-        "//toolchain/diagnostics:diagnostic_emitter",
+        "//toolchain/diagnostics:emitter",
         "//toolchain/diagnostics:file_diagnostics",
         "@llvm-project//llvm:Core",
         "@llvm-project//llvm:MC",

toolchain/codegen/codegen.cpp

@@ -13,7 +13,7 @@
 #include "llvm/MC/TargetRegistry.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/TargetParser/Host.h"
-#include "toolchain/diagnostics/diagnostic_consumer.h"
+#include "toolchain/diagnostics/consumer.h"
 
 namespace Carbon {
 

toolchain/codegen/codegen.h

@@ -7,7 +7,7 @@
 
 #include "llvm/IR/Module.h"
 #include "llvm/Target/TargetMachine.h"
-#include "toolchain/diagnostics/diagnostic_consumer.h"
+#include "toolchain/diagnostics/consumer.h"
 #include "toolchain/diagnostics/file_diagnostics.h"
 
 namespace Carbon {