Call cmake_minimum_required at the top of CMakeLists.txt

Summary:
Since r298413, the NEW behavior of the CMake policy CMP0056 is followed.
However, it is only effective after the call to cmake_minimum_required.
This causes CMAKE_EXE_LINKER_FLAGS etc. to be unused when CMake tries to
check compilers for languages specified in the 'project' declaration.

Set cmake_minimum_required(VERSION) at the top of the file and ahead of
the project declaration.

Reviewers: beanz

Subscribers: mgorny, srhines, llvm-commits

Differential Revision: https://reviews.llvm.org/D34282

llvm-svn: 305593