Fix -gdb-exit to detach if was attached or destroy otherwise (MI)

Summary:
This patch fixes -gdb-exit for locally target. It includes the following changes:
# Fix Process::Finalize
# Use SBProcess::Destroy in -gdb-exit

Reviewers: abidh, zturner, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits, clayborg, abidh

Differential Revision: http://reviews.llvm.org/D8298

llvm-svn: 233255