Fix and simplify PrepareCommandsForSourcing

Spotted some problems in the Driver's PrepareCommandsForSourcing while
helping a colleague track another problem.

1. One error case was not handled because there was no else clause.
Fixed by switching to llvm's early-out style instead of nested
`if (succes) { } else { }` cases.  This keeps error handling close
to the actual error.

2. One call-site failed to call the clean-up function.  I solved this
by simplifying the API.  PrepareCommandsForSourcing no longer requires
the caller to provide a buffer for the pipe's file descriptors and to
call a separate clean-up function later.  PrepareCommandsForSourcing
now ensures the file descriptors are handled before returning.
(The read end of the pipe is held open by the returned FILE * as
before.)

I also eliminated an unnecessary local, shorted the lifetime of another,
and tried to improve the comments.

I wrapped the call to open the pipe to get the `#ifdef`s out of the
mainline.  I replaced the `close`/`_close` calls with a platform-neutral
helper from `llvm::sys` for the same reason.  Per discussion on the
review, I'm leaving the `fdopen` call to use the spelling that Windows
has officially deprecated because it still works it avoids more `#ifdef`s.

Differential Revision: https://reviews.llvm.org/D60152

llvm-svn: 357626