Filter out 0-length tokens when generating syntax model nodes.

Resolves rdar://problem/33601932.

Author: Nathan Hawes

lib/IDE/SyntaxModel.cpp

@@ -63,6 +63,9 @@ SyntaxModelContext::SyntaxModelContext(SourceFile &SrcFile)
   auto LiteralStartLoc = Optional<SourceLoc>();
   for (unsigned I = 0, E = Tokens.size(); I != E; ++I) {
     auto &Tok = Tokens[I];
+    // Ignore empty string literals between interpolations, e.g. "\(1)\(2)"
+    if (!Tok.getLength())
+        continue;
     SyntaxNodeKind Kind;
     SourceLoc Loc;
     Optional<unsigned> Length;

test/IDE/coloring.swift

@@ -240,6 +240,9 @@ func f(x: Int) -> Int {
   """
       This is a multiline\( "interpolated" )string
    """
+
+  // CHECK: <str>"</str>\<anchor>(</anchor><int>1</int><anchor>)</anchor>\<anchor>(</anchor><int>1</int><anchor>)</anchor><str>"</str>
+  "\(1)\(1)"
 }
 
 // CHECK: <kw>func</kw> bar(x: <type>Int</type>) -> (<type>Int</type>, <type>Float</type>) {