Move the execution code from Compilation::ExecuteCommand to Command::Execute

I think it makes sense that a Command knows how to execute itself.

There's no functionality change but i rewrote the code to avoid the manual
memory management of Argv.

My motivation for this is that I plan to subclass Command to build fall-back
functionality into clang-cl.

Differential Revision: http://llvm-reviews.chandlerc.com/D1654