Add -gdb-show and target-async option in -gdb-show command (MI)

Summary:
This path adds -gdb-show command with 1 option: target-async.

Also it adds tests for -gdb-set and -gdb-show commands.

All tests pass on OS X.

Test Plan: ./dotest.py -v --executable $BUILDDIR/bin/lldb -f MiGdbSetShowTestCase

Reviewers: clayborg, abidh

Reviewed By: clayborg, abidh

Subscribers: lldb-commits, clayborg, abidh

Differential Revision: http://reviews.llvm.org/D8566

llvm-svn: 233114