Create a structure to define the features from to_llvm_features.

Rather than returning an array of features from to_llvm_features, return a structure that contains
the dependencies. This also contains metadata on how the features depend on each other to allow for
the correct enabling and disabling.

compiler/rustc_codegen_llvm/src/lib.rs

@@ -10,6 +10,7 @@
 #![feature(iter_intersperse)]
 #![feature(let_chains)]
 #![feature(never_type)]
+#![feature(impl_trait_in_assoc_type)]
 #![recursion_limit = "256"]
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]