[CMake] Make LLVM_VERSION_* variables user definable

CMake's set command overwrites existing values. Package maintainers may want or need to set the version variables manually, so we need to only set them if they are not already defined. Note I use the "if(NOT DEFINED ...)" syntax deliberately in the last case because empty string is a valid value for the suffx, but not the other variables.

llvm-svn: 250333