Set cmake policy CMP0075 to NEW

Summary:
The policy is about cmake_include_files ignoring
CMAKE_REQUIRED_LIBRARIES in the OLD behavior. Llvm already sets this
policy to NEW, but that is overridden by our cmake_minimum_required
command.

This makes our cmake policy setup consistent with the llvm build files
and avoids a warning when using newer versions of cmake.

Reviewers: sgraenitz, xiaobai

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D58279

llvm-svn: 354251