[lldb/CMake] Use PYTHON_HOME as a hint to find Python 3.

[JDevlieghere]

Summary

http://lists.llvm.org/pipermail/lldb-dev/2020-February/015998.html

[amccarth]

Thanks for helping figure this out!

lldb/cmake/modules/FindPythonInterpAndLibs.cmake

 #.rst:
 # FindPythonInterpAndLibs
 # -----------
 #
 # Find the python interpreter and libraries as a whole.
 
 if(PYTHON_LIBRARIES AND PYTHON_INCLUDE_DIRS AND PYTHON_EXECUTABLE AND SWIG_EXECUTABLE)
   set(PYTHONINTERPANDLIBS_FOUND TRUE)
 else()
   find_package(SWIG 2.0)
   if (SWIG_FOUND)
     if ("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
+      # Use PYTHON_HOME as a hint to find Python 3.
+      set(Python3_ROOT_DIR PYTHON_HOME)
       find_package(Python3 COMPONENTS Interpreter Development)
       if (Python3_FOUND AND Python3_Interpreter_FOUND)
         set(PYTHON_LIBRARIES ${Python3_LIBRARIES})
         set(PYTHON_INCLUDE_DIRS ${Python3_INCLUDE_DIRS})
         set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})
         mark_as_advanced(
           PYTHON_LIBRARIES
           PYTHON_INCLUDE_DIRS