Fix Python binding generation build step on Windows

Summary:
If Python is installed to a location that contains spaces
(e.g. "C:\Program Files\Python3") then the build fails while attempting
to run the modify-python-lldb.py script because the path to the Python
executable is not double-quoted before being passed to the shell. The
fix consists of letting Python handle the formatting of the command
line, since subprocess.Popen() is perfectly capable of handling paths
containing spaces if it's given the command and arguments as a list
instead of a single pre-formatted string.

Reviewers: zturner, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D25396

llvm-svn: 284100