[lldb] build failure for LLDB_PYTHON_EXE_RELATIVE_PATH on greendragon

see: https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/38387/console

```
Could not find a relative path to sys.executable under sys.prefix
tried: /usr/local/opt/python/bin/python3.7
tried: /usr/local/opt/python/bin/../Frameworks/Python.framework/Versions/3.7/bin/python3.7
sys.prefix: /usr/local/Cellar/python/3.7.1/Frameworks/Python.framework/Versions/3.7
```

It was unable to find LLDB_PYTHON_EXE_RELATIVE_PATH because it was not resolving
the real path of sys.prefix.

caused by: https://reviews.llvm.org/D113650