Rollup merge of #137549 - oli-obk:llvm-ffi, r=davidtwco

Clean up various LLVM FFI things in codegen_llvm

cc ```@ZuseZ4``` I touched some autodiff parts

The major change of this PR is [bfd88ce](bfd88cead0) which makes `CodegenCx` generic just like `GenericBuilder`

The other commits mostly took advantage of the new feature of making extern functions safe, but also just used some wrappers that were already there and shrunk unsafe blocks.

best reviewed commit-by-commit

compiler/rustc_codegen_llvm/src/common.rs

@@ -1,5 +1,7 @@
 //! Code that is useful in various codegen modules.
 
+use std::borrow::Borrow;
+
 use libc::{c_char, c_uint};
 use rustc_abi as abi;
 use rustc_abi::Primitive::Pointer;
@@ -18,6 +20,7 @@ use tracing::debug;
 
 use crate::consts::const_alloc_to_llvm;
 pub(crate) use crate::context::CodegenCx;
+use crate::context::{GenericCx, SCx};
 use crate::llvm::{self, BasicBlock, Bool, ConstantInt, False, Metadata, True};
 use crate::type_::Type;
 use crate::value::Value;
@@ -81,7 +84,7 @@ impl<'ll> Funclet<'ll> {
     }
 }
 
-impl<'ll> BackendTypes for CodegenCx<'ll, '_> {
+impl<'ll, CX: Borrow<SCx<'ll>>> BackendTypes for GenericCx<'ll, CX> {
     type Value = &'ll Value;
     type Metadata = &'ll Metadata;
     // FIXME(eddyb) replace this with a `Function` "subclass" of `Value`.
@@ -118,7 +121,7 @@ impl<'ll> CodegenCx<'ll, '_> {
     }
 }
 
-impl<'ll, 'tcx> ConstCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
+impl<'ll, 'tcx> ConstCodegenMethods for CodegenCx<'ll, 'tcx> {
     fn const_null(&self, t: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMConstNull(t) }
     }
@@ -342,7 +345,7 @@ impl<'ll, 'tcx> ConstCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         }
     }
 
-    fn const_data_from_alloc(&self, alloc: ConstAllocation<'tcx>) -> Self::Value {
+    fn const_data_from_alloc(&self, alloc: ConstAllocation<'_>) -> Self::Value {
         const_alloc_to_llvm(self, alloc, /*static*/ false)
     }