[clang][deps] Fix handling of -MT in module command-line

Follow-up to 6626f6fe, this fixes the handling of -MT
* If no targets are provided, we need to invent one since cc1 expects
  the driver to have handled it. The default is to use -o, quoting as
  necessary for a make target.
* Fix the splitting for empty string, which was incorrectly treated as
  {""} instead of {}.
* Add a way to test this behaviour in clang-scan-deps.

Differential Revision: https://reviews.llvm.org/D129607