clang-format: Fix assertion on incomplete string literals.

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

llvm-svn: 191639
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 08b5643..833a87c 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -3398,6 +3398,11 @@
   verifyFormat(
       "llvm::errs() << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\n"
       "                    .aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa();");
+
+  // Incomplete string literal.
+  EXPECT_EQ("llvm::errs() << \"\n"
+            "             << a;",
+            format("llvm::errs() << \"\n<<a;"));
 }
 
 TEST_F(FormatTest, UnderstandsEquals) {