clang-format: Narrow down raw string literal line break exception.

For multiline raw string literals, we generally want to respect the
author's choice of linebreak before the 'R"(' as the rest of the raw
string might be aligned to it and we cannot (commonly) modify the
content.

For single-line raw string literals, this doesn't make any sense and so
we should just treat them as regular string literals in this regard.

llvm-svn: 328201