clang-cl: Don't store the cl compiler Tool on the stack (PR20131)

The Command will refer back to the Tool as its source,
so it has to outlive the Command.

Having the Tool on the stack would cause us to crash
when using "clang-cl -GR -fallback", because if the
Command fails, Driver::ExecuteCompilation tries to
peek at the Command's source.

Differential Revision: http://reviews.llvm.org/D4314