[Target] Move InferiorCall to Process

Summary:
InferiorCall is only ever used in Process, and it is not specific to
POSIX. By moving it to Process, we can remove all dependencies on plugins from
Process. Moving InferiorCall to Process seems to achieve this quite well.
Additionally, the name InferiorCall is a little vague now, so we rename
it something a bit more specific.

Reviewers: JDevlieghere, clayborg, compnerd, labath

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67472

llvm-svn: 371796