Follow-up to 6626f6fec3, 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
Ben Langmuir