Rollup merge of #60097 - cuviper:llvm8-mergefunc-use-aliases, r=rkruppe

Use -mergefunc-use-aliases for any LLVM >= 8

This functionality is not specific to Rust's LLVM, but any starting in LLVM 8.0,
as noted in <https://github.com/rust-lang/rust/pull/56358#discussion_r237702197>.

cc @nikic
r? @rkruppe

src/librustc_codegen_llvm/llvm_util.rs

@@ -61,7 +61,7 @@ unsafe fn configure_llvm(sess: &Session) {
         if sess.opts.debugging_opts.disable_instrumentation_preinliner {
             add("-disable-preinline");
         }
-        if llvm::LLVMRustIsRustLLVM() {
+        if get_major_version() >= 8 {
             match sess.opts.debugging_opts.merge_functions
                   .unwrap_or(sess.target.target.options.merge_functions) {
                 MergeFunctions::Disabled |