[ScriptInterpreter] Make sure that PYTHONHOME is right.

Summary:
For the only version of Python actually supported on Darwin.

<rdar://problem/40961425>

Reviewers: jingham, friss, JDevlieghere, aprantl, jasonmolenda

Subscribers: jdoerfert, llvm-commits, lldb-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59719

llvm-svn: 356816