clang-format: Fix assertion on incomplete string literals.

Before, this could would lead to an assert:
  llvm::errs() << "
               << a;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191639 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Format/TokenAnnotator.cpp b/lib/Format/TokenAnnotator.cpp
index 992104f..6772c89 100644
--- a/lib/Format/TokenAnnotator.cpp
+++ b/lib/Format/TokenAnnotator.cpp
@@ -1161,7 +1161,11 @@
   if (Right.is(tok::lessless)) {
     if (Left.is(tok::string_literal)) {
       StringRef Content = Left.TokenText;
-      Content = Content.drop_back(1).drop_front(1).trim();
+      if (Content.startswith("\""))
+        Content = Content.drop_front(1);
+      if (Content.endswith("\""))
+        Content = Content.drop_back(1);
+      Content = Content.trim();
       if (Content.size() > 1 &&
           (Content.back() == ':' || Content.back() == '='))
         return 25;