clang-format: Fix alignment of ObjC string literals.

This used to interfere with AlwaysBreakBeforeMultilineStrings.

This fixes llvm.org/PR17856.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194310 91177308-0d34-0410-b5e6-96231b3b80d8