[CodeCompletion] Fix an assertion violation. rdar://23306548

When creating a SequenceExpr, we should always have an odd number of
elements.

lib/AST/Expr.cpp

@@ -899,6 +899,7 @@ bool OverloadSetRefExpr::hasBaseObject() const {
 }
 
 SequenceExpr *SequenceExpr::create(ASTContext &ctx, ArrayRef<Expr*> elements) {
+  assert(elements.size() & 1 && "even number of elements in sequence");
   void *Buffer = ctx.Allocate(sizeof(SequenceExpr) +
                               elements.size() * sizeof(Expr*),
                               alignof(SequenceExpr));

lib/Parse/ParseExpr.cpp

@@ -291,6 +291,10 @@ parse_operator:
           CodeCompletion->completeAssignmentRHS(assign);
         }
         consumeToken();
+        if (SequencedExprs.size() > 0 && (SequencedExprs.size() & 1) == 0) {
+          // Make sure we have odd number of sequence exprs.
+          SequencedExprs.pop_back();
+        }
         auto Result = SequencedExprs.size() == 1 ?
           makeParserResult(SequencedExprs[0]):
           makeParserResult(SequenceExpr::create(Context, SequencedExprs));

test/SourceKit/CodeComplete/complete_crash1.swift

@@ -0,0 +1,8 @@
+// RUN: %complete-test -tok=TOK1 -hide-none %s -- %s
+
+import QuartzCore
+
+class Cl{
+  var L : CALayer = {
+    let layer = CALayer()
+    layer.transform = #^TOK1^#