Make it explicit that the only entry points to the Program object are

Make it explicit that the only entry points to the Program object are
through the static helper functions. This is already true throughout the
codebase.

Slowly, I'm going to re-implement these static helpers in terms of a new
process based interface which can expose more information, and remove
the program object entirely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171335 91177308-0d34-0410-b5e6-96231b3b80d8