clang/Modules: Sink ASTReadResult in ReadControlBlock, NFC

Simplify the code by avoiding some state that wasn't being used.  The
function-level `Result` was only assigned a value other than `Success`
in the handler for `OPTIONS_BLOCK_ID`, but in that case it also hits an
early return.  Remove it at the function-level to make it obvious that
the normal case always returns `Success`.
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 9111b60..36c2952 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -2561,7 +2561,6 @@
                             const ModuleFile *ImportedBy,
                             unsigned ClientLoadCapabilities) {
   BitstreamCursor &Stream = F.Stream;
-  ASTReadResult Result = Success;
 
   if (llvm::Error Err = Stream.EnterSubBlock(CONTROL_BLOCK_ID)) {
     Error(std::move(Err));
@@ -2652,7 +2651,7 @@
         }
       }
 
-      return Result;
+      return Success;
     }
 
     case llvm::BitstreamEntry::SubBlock:
@@ -2682,9 +2681,10 @@
           bool AllowCompatibleConfigurationMismatch =
               F.Kind == MK_ExplicitModule || F.Kind == MK_PrebuiltModule;
 
-          Result = ReadOptionsBlock(Stream, ClientLoadCapabilities,
-                                    AllowCompatibleConfigurationMismatch,
-                                    *Listener, SuggestedPredefines);
+          ASTReadResult Result =
+              ReadOptionsBlock(Stream, ClientLoadCapabilities,
+                               AllowCompatibleConfigurationMismatch, *Listener,
+                               SuggestedPredefines);
           if (Result == Failure) {
             Error("malformed block record in AST file");
             return Result;