clang-format: Improve handling of raw string literals.

Especially try to keep existing line breaks before raw string literals,
as the code author might have aligned content to it.

Thereby, clang-format now keeps things like:

  parseStyle(R"(
      BasedOnStyle: Google,
      ColumnLimit: 100)");
  parseStyle(
      R"(BasedOnStyle: Google,
         ColumnLimit: 100)");

llvm-svn: 197368