[CMake] Move project() call to main CMake file

Summary:
The main CMake file don't have a project() call. In this case, cmake will run a dummy project(Project ) at the very beginning. Even before cmake_minimum_required. And a series of compiler detections will be triggered.

This is problematic if we depends on some policy to be set. E.g. CMP0056. try_compile will fail before we have a chance to do anything.

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65362

llvm-svn: 367273