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_session/src/options.rs

@@ -1579,6 +1579,9 @@ options! {
         "in general, enable more debug printouts (default: no)"),
     verify_llvm_ir: bool = (false, parse_bool, [TRACKED],
         "verify LLVM IR (default: no)"),
+    virtual_function_elimination: bool = (false, parse_bool, [TRACKED],
+        "enables dead virtual function elimination optimization. \
+        Requires `-Clto[=[fat,yes]]`"),
     wasi_exec_model: Option<WasiExecModel> = (None, parse_wasi_exec_model, [TRACKED],
         "whether to build a wasi command or reactor"),