clang-format: Fix string breaking after "<<".

Before, clang-format would not break overly long string literals
following a "<<" with FormatStyle::AlwaysBreakBeforeMultilineStrings
being set.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187650 91177308-0d34-0410-b5e6-96231b3b80d8