Add -Zvirtual-function-elimination flag

Adds the virtual-function-elimination unstable compiler flag and a check
that this flag is only used in combination with -Clto. LLVM can only
apply this optimization with fat LTO.

compiler/rustc_interface/src/tests.rs

@@ -797,6 +797,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(unleash_the_miri_inside_of_you, true);
     tracked!(use_ctors_section, Some(true));
     tracked!(verify_llvm_ir, true);
+    tracked!(virtual_function_elimination, true);
     tracked!(wasi_exec_model, Some(WasiExecModel::Reactor));
 
     macro_rules! tracked_no_crate_hash {