[CMake] Cleanup CMake version checking for 2.8.12
[CMake] Cleanup CMake version checking for 2.8.12

Now that we're on CMake 3.4.3 this stuff isn't needed anymore.

The cmake_2_8_12_* variables were unused in Compiler-RT, and the policies being set to NEW are all NEW by default now.

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@272313 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt