Start sketching out a roadmap for better subprocess management in the

Start sketching out a roadmap for better subprocess management in the
LLVM libraries. Also, clean up the doxygen and formatting of the
existing interfaces.

With this change I'm calling the existing interface "legacy" because I'd
like to replace it with something much better. My end goal is to expose
a common set of interfaces for inspecting various properties of
a process, and implementations to expose those both for the current
process and for child processes. This will also expose more rich
interfaces for spawning and controling a subprocess, notably to use
system calls like wait3 and wait4 where available and gather detailed
resource usage stats about the subprocess.

My plan (discussed with Michael Spencer on IRC) is to base this loosely
around the proposed Boost.Process interface, but to implement
a relatively small subset of that functionality based around the needs
of LLVM, Clang, the Clang driver, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171285 91177308-0d34-0410-b5e6-96231b3b80d8