[CMake]Getting rid of references to LLVM_SUBMIT_VERSION and LLVM_SUBMIT_SUBVERSION in favor of LLVM_VERSION_MAJOR and LLVM_VERSION_MINOR.

LLVM_SUBMIT_VERSION and LLVM_SUBMIT_SUBVERSION are commonly used variable names in the autoconf build system. They seem to have leeched into clang's CMake in a few places, and should just be replaced by the LLVM_VERSION_* variables where appropriate.

llvm-svn: 250246