[cmake] Set CMP0075 to NEW

Make the check_include_file* macros honor CMAKE_REQUIRED_LIBRARIES. This
shouldn't cause any of the configuration checks to give different
results (and I did clean configures before and after this change and
confirmed that the resulting CMake caches were identical, though of
course that's just one machine). This suppresses a warning when building
with CMake 3.12 or later.

This doesn't suppress the warning in clang, because clang does its own
cmake_minimum_required call even when being built in-tree, and that
resets all policy settings. I'll address that separately.

Differential Revision: https://reviews.llvm.org/D54236

llvm-svn: 346377