clang-format: Better support for multi-line wide string literals.

Before:
  SomeFunction(L"A" L"B");

After:
  SomeFunction(L"A"
               L"B");

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197785 91177308-0d34-0410-b5e6-96231b3b80d8