[cmake] Remove policy conditionals

LLVM now requires a minimum of cmake 3.4.3, and all the policies
currently being set are present in that cmake version, so the
conditionals will always be true and are therefore unnecessary. The
movation is that the conditionals can give the false impression that the
policy settings are optional, whereas for example it's necessary to set
CMP0056 in order for `check_linker_flags` to operate correctly after
r316972. Inline the project version and language setting in the process.

Differential Revision: https://reviews.llvm.org/D39442

llvm-svn: 317264