Renaming change: 41 -> 45 and 4.1 -> 4.5

OpenMP 4.1 is now OpenMP 4.5.  Any mention of 41 or 4.1 is replaced with
45 or 4.5.  Also, if the CMake option LIBOMP_OMP_VERSION is 41, CMake warns that
41 is deprecated and to use 45 instead.

llvm-svn: 272687