[Parse] Preserve original member hashing behaviour

A previous commit inadvertently changed the
logic such that the member hash of an extension
body would be set to a partial interface hash.
Luckily this shouldn't have caused any behavioural
change as the interface hash itself would have
been left unaffected.

This commit makes sure we preserve the original
behaviour where if we don't have the body tokens
hashed separately, we give the body hash a default
constructed MD5.

Noticed by inspection.

Author: hamishknight

lib/Parse/ParseDecl.cpp

@@ -4471,11 +4471,11 @@ Parser::parseDeclList(SourceLoc LBLoc, SourceLoc &RBLoc, Diag<> ErrorDiag,
 
   // If we're hashing the type body separately, record the curly braces but
   // nothing inside for the interface hash.
-  Optional<llvm::SaveAndRestore<Optional<llvm::MD5>>> S;
+  Optional<llvm::SaveAndRestore<Optional<llvm::MD5>>> MemberHashingScope;
   if (IDC->areTokensHashedForThisBodyInsteadOfInterfaceHash()) {
     recordTokenHash("{");
     recordTokenHash("}");
-    S.emplace(CurrentTokenHash, llvm::MD5());
+    MemberHashingScope.emplace(CurrentTokenHash, llvm::MD5());
   }
 
   std::vector<Decl *> decls;
@@ -4511,8 +4511,8 @@ Parser::parseDeclList(SourceLoc LBLoc, SourceLoc &RBLoc, Diag<> ErrorDiag,
     return std::make_pair(decls, None);
 
   llvm::MD5::MD5Result result;
-  auto tokenHashForThisDeclList = CurrentTokenHash.getValueOr(llvm::MD5());
-  tokenHashForThisDeclList.final(result);
+  auto declListHash = MemberHashingScope ? *CurrentTokenHash : llvm::MD5();
+  declListHash.final(result);
   llvm::SmallString<32> tokenHashString;
   llvm::MD5::stringifyResult(result, tokenHashString);
   return std::make_pair(decls, tokenHashString.str().str());