Move FileAction, ProcessInfo and ProcessLaunchInfo from Target to Host

Summary:
These classes describe the details of the process we are about to
launch, and so they are naturally used by the launching code in the Host
module. Previously they were present in Target because that is the most
important (but by far not the only) user of the launching code.

Since the launching code has other customers, must of which do not care
about Targets, it makes sense to move these classes to the Host layer,
next to the launching code.

This move reduces the number of times that Target is included from host
to 8 (it used to be 14).

Reviewers: zturner, clayborg, jingham, davide, teemperor

Subscribers: emaste, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D56602

llvm-svn: 353047