Remove the trailing whitespace of formatted lines.

So far, clang-format has always assumed the whitespace belonging to the
subsequent token. This has the negative side-effect that when
clang-format formats a line, it does not remove its trailing whitespace,
as it belongs to the next token.

Thus, this patch fixes most of llvm.org/PR15062.

We are not zapping a file's trailing whitespace so far, as this does not
belong to any token we see during formatting. We need to fix this in a
subsequent patch.