Function LLVMRustVersionMajor 

pub(crate) unsafe extern "C" fn LLVMRustVersionMajor() -> u32

Returns the LLVM major version that the compiler was built with.

Note that this is hard-coded as LLVM_VERSION_MAJOR when RustWrapper.cpp is built. This could be different than what the runtime LLVM library reports in LLVMGetVersion, so we assert their equality in configure_llvm.