[Gardening] Update clang-function-type-serialization comments.

We have landed support for serialization Clang function types, but there is
still work to be done here.

Moreover, we should use a consistent style with `[(FIXME|TODO|NOTE): label]`,
instead of using different styles throughout the code.

Author: varungandhi-apple

include/swift/AST/PrintOptions.h

@@ -327,8 +327,7 @@ struct PrintOptions {
   };
 
   /// Whether to print function @convention attribute on function types.
-  // FIXME: [clang-function-type-serialization] Once we start serializing Clang
-  // types, we should also start printing the full type in the swiftinterface.
+  // [TODO: Clang-type-plumbing] Print the full type in the swiftinterface.
   FunctionRepresentationMode PrintFunctionRepresentationAttrs =
     FunctionRepresentationMode::NameOnly;
 

include/swift/AST/Types.h

@@ -3013,8 +3013,7 @@ class AnyFunctionType : public TypeBase {
     static void assertIsFunctionType(const clang::Type *);
 
     ExtInfo(unsigned Bits, Uncommon Other) : Bits(Bits), Other(Other) {
-      // TODO: [clang-function-type-serialization] Once we start serializing
-      // the Clang type, we should also assert that the pointer is non-null.
+      // [TODO: Clang-type-plumbing] Assert that the pointer is non-null.
       auto Rep = Representation(Bits & RepresentationMask);
       if ((Rep == Representation::CFunctionPointer) && Other.ClangFunctionType)
         assertIsFunctionType(Other.ClangFunctionType);

include/swift/Basic/LangOptions.h

@@ -274,8 +274,7 @@ namespace swift {
     /// Use Clang function types for computing canonical types.
     /// If this option is false, the clang function types will still be computed
     /// but will not be used for checking type equality.
-    /// FIXME: [clang-function-type-serialization] This option should be turned
-    /// on once we start serializing clang function types.
+    /// [TODO: Clang-type-plumbing] Turn on for feature rollout.
     bool UseClangFunctionTypes = false;
 
     /// Whether to use the import as member inference system

include/swift/Frontend/ModuleInterfaceSupport.h

@@ -33,7 +33,7 @@ struct ModuleInterfaceOptions {
   bool PreserveTypesAsWritten = false;
 
   /// Should we emit the cType when printing @convention(c) or no?
-  /// FIXME: [clang-function-type-serialization] This check should go away.
+  /// [TODO: Clang-type-plumbing] This check should go away.
   bool PrintFullConvention = false;
 
   /// Copy of all the command-line flags passed at .swiftinterface

lib/AST/ASTPrinter.cpp

@@ -4019,8 +4019,7 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
         break;
       case SILFunctionType::Representation::CFunctionPointer:
         Printer << "c";
-        // FIXME: [clang-function-type-serialization] Once we start serializing
-        // Clang function types, we should be able to remove the second check.
+        // [TODO: Clang-type-plumbing] Remove the second check.
         if (printNameOnly || !info.getUncommonInfo().hasValue())
           break;
         printCType(Ctx, Printer, info);
@@ -4086,8 +4085,7 @@ class TypePrinter : public TypeVisitor<TypePrinter> {
         break;
       case SILFunctionType::Representation::CFunctionPointer:
         Printer << "c";
-        // FIXME: [clang-function-type-serialization] Once we start serializing
-        // Clang function types, we should be able to remove the second check.
+        // [TODO: Clang-type-plumbing] Remove the second check.
         if (printNameOnly || !info.getUncommonInfo().hasValue())
           break;
         printCType(Ctx, Printer, info);