Always break after multi-line string literals.

Otherwise, other parameters can be quite hidden.
Reformatted unittests/Format/FormatTest.cpp after this.

Before:
someFunction("Always break between multi-line"
             " string literals", and, other, parameters);

After:
someFunction("Always break between multi-line"
             " string literals",
             and, other, parameters);

llvm-svn: 175436