Allow specifying an exit code for the 'quit' command

Summary:
This patch adds the possibility to specify an exit code when calling quit.
We accept any int, even though it depends on the user what happens if the int is
out of the range of what the operating system supports as exit codes.

Fixes rdar://problem/38452312

Reviewers: davide, jingham, clayborg

Reviewed By: jingham

Subscribers: clayborg, jingham, lldb-commits

Differential Revision: https://reviews.llvm.org/D48659

llvm-svn: 336824