Use LLVM_VERSION_MAJOR instead as the maximum allowed value. This change
is needed to fix regression tests that fail when vendors set
CLANG_VERSION_MAJOR to a value that is lower than LLVM_VERSION_MAJOR
when building the compiler.
For example, clang/test/CodeGenCXX/mangle-concept.cpp fails with the
following error if -DCLANG_VERSION_MAJOR=17 is passed to cmake:
invalid value '19' in '-fclang-abi-compat=19'