[PATCH] [lld][Driver] Add maximum concurrency option.

[Sean Silva]

  > Passing this as a parameter is weird, as it would only have an effect on the first call.

  Hm, maybe I'm not understanding this correctly. Is `--thread-count` commandline argument supposed to cap the number of threads of one linker invocation, or cap the number of threads for all the linker instances running in the current process? From the name, it sounds like the former, but the implementation suggests the latter.

  Concretely, suppose that a client tries to run two linker invocations in the same process, and wants each of them to get 2 threads, for a total of 4 threads. Would it work for the client to specify `--thread-count=2` for each linker invocation? If not, then how can that behavior be accomplished?

  Relatedly, is the intention that an instance of the "Executor" class be global to the process, or created piecemeal for particular tasks?

http://llvm-reviews.chandlerc.com/D698