Componentize lldb/scripts to use with LLVM_DISTRIBUTION_COMPONENTS

Summary:
I'd like to install lldb using the install-distribution target with LLVM_DISTRIBUTION_COMPONENTS but this is currently not possible as the lldb/scripts do not provide any component we can use and install the python scripts.
For this effect I created an lldb-python-scripts target and added the install-lldb-python-scripts llvm install target.

I tested with:
cmake ... -DLLVM_ENABLE_PROJECTS="clang;lldb" -DLLVM_DISTRIBUTION_COMPONENTS="lldb;liblldb;lldb-python-scripts" ...
DESTDIR=... ninja install-distribution

Then checked with bin/lldb -x -o 'script import lldb'

Reviewers: labath, xiaobai, clayborg, lanza

Subscribers: mgorny, lldb-commits, smeenai, wallace

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68370

llvm-svn: 373768