Add LLVM module flags required for the VFE opt

To apply the optimization the `Virtual Function Elim` module flag has to
be set. To apply this optimization post-link the `LTOPostLink` module
flag has to be set.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1936,6 +1936,7 @@ extern "C" {
         name: *const c_char,
         value: u32,
     );
+    pub fn LLVMRustHasModuleFlag(M: &Module, name: *const c_char, len: size_t) -> bool;
 
     pub fn LLVMRustMetadataAsValue<'a>(C: &'a Context, MD: &'a Metadata) -> &'a Value;