[CMake][libcxx] Move Python check to main CMake file

This is to make sure this check is called even when building as
part of LLVM runtimes when we are doing standalone but not out of
tree build.

Differential Revision: https://reviews.llvm.org/D28392

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291592 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed