[IDE] Make sure to handle the case of virtual-only attributes

for syntax coloring.
Add a test for the above.
This addresses <rdar://problem/16584438>.

Swift SVN r16194

lib/IDE/SyntaxModel.cpp

@@ -488,7 +488,7 @@ bool ModelASTWalker::handleAttrs(const TypeAttributes &Attrs) {
 }
 bool ModelASTWalker::handleAttrLocs(SourceLoc BeginLoc,
                                     ArrayRef<SourceLoc> Locs) {
-  if (BeginLoc.isInvalid() || Locs.empty())
+  if (Locs.empty())
     return true;
 
   SmallVector<SourceLoc, 6> SortedLocs(Locs.begin(), Locs.end());
@@ -499,6 +499,12 @@ bool ModelASTWalker::handleAttrLocs(SourceLoc BeginLoc,
   );
   Locs = SortedLocs;
 
+  // The BeginLoc is invalid if there's no leading '@', which happens if there
+  // are only virtual attributes. In that case we'll use the location of the
+  // first attribute.
+  if (BeginLoc.isInvalid())
+    BeginLoc = Locs.front();
+
   std::vector<Token> Toks =
       swift::tokenize(LangOpts, SM, BufferID,
                       SM.getLocOffsetInBuffer(BeginLoc, BufferID),