[Fix #1474] LineEndConcatenation cop catches multiline string with '<<' and '\'

Iterates through children of nodes without valid source map beginnings and verifies they are string types.
 `node.loc.begin` for nodes of type `dstr`will sometimes (always?) return `nil`.
This causes any string containing a concatenation with `\\` to pass this cop, as seen in #1474.
If the first test fails, this will iterate through the node's children and recursively check that they all pass it.