Drop SerializedOK parameter from getRawComment

It doesn't seem like there's any client that's
actually taking advantage of setting it to `false`,
and its default value of `false` is more likely
than not going to cause clients to accidentally
miss comments that they may want. In fact, this
was exactly the case for code completion's brief
field. Finally, the parameter wasn't even
consistently applied, as we would attempt to
deserialize swiftdoc comments even if it were
`false`.

lib/IDE/ModuleInterfacePrinting.cpp

@@ -829,7 +829,7 @@ static SourceLoc getDeclStartPosition(SourceFile &File) {
   for (auto D : File.getTopLevelDecls()) {
     if (tryUpdateStart(D->getStartLoc())) {
       tryUpdateStart(D->getAttrs().getStartLoc());
-      auto RawComment = D->getRawComment(/*SerializedOK=*/false);
+      auto RawComment = D->getRawComment();
       if (!RawComment.isEmpty())
         tryUpdateStart(RawComment.Comments.front().Range.getStart());
     }