Remove inherent methods from several LLVM FFI types

Using a helper trait allows the conversions to remain in `rustc_codegen_llvm`,
even if the FFI types are moved to a different crate.

compiler/rustc_codegen_llvm/src/declare.rs

@@ -26,7 +26,7 @@ use crate::abi::FnAbiLlvmExt;
 use crate::common::AsCCharPtr;
 use crate::context::{CodegenCx, GenericCx, SCx, SimpleCx};
 use crate::llvm::AttributePlace::Function;
-use crate::llvm::Visibility;
+use crate::llvm::{FromGeneric, Visibility};
 use crate::type_::Type;
 use crate::value::Value;
 use crate::{attributes, llvm};