[CMake] Python bindings generation polishing

Summary:
Simplify SWIG invocation and handling of generated files.

The `swig_wrapper` target can generate `LLDBWrapPython.cpp` and `lldb.py` in its own binary directory, so we can get rid of a few global variables and their logic. We can use the swig_wrapper's BINARY_DIR target property to refer to it and liblldb's LIBRARY_OUTPUT_DIRECTORY to refer to the framework/shared object output directory.

Reviewers: JDevlieghere, aprantl, stella.stamenova, beanz, zturner, xiaobai

Reviewed By: aprantl

Subscribers: mgorny, lldb-commits, #lldb

Differential Revision: https://reviews.llvm.org/D55332

llvm-svn: 350393