codegen: use new {re,de,}allocator annotations in llvm

This obviates the patch that teaches LLVM internals about
_rust_{re,de}alloc functions by putting annotations directly in the IR
for the optimizer.

The sole test change is required to anchor FileCheck to the body of the
`box_uninitialized` method, so it doesn't see the `allocalign` on
`__rust_alloc` and get mad about the string `alloca` showing up. Since I
was there anyway, I added some checks on the attributes to prove the
right attributes got set.

While we're here, we also emit allocator attributes on
__rust_alloc_zeroed. This should allow LLVM to perform more
optimizations for zeroed blocks, and probably fixes #90032. [This
comment](https://github.com/rust-lang/rust/issues/24194#issuecomment-308791157)
mentions "weird UB-like behaviour with bitvec iterators in
rustc_data_structures" so we may need to back this change out if things
go wrong.

The new test cases require LLVM 15, so we copy them into LLVM
14-supporting versions, which we can delete when we drop LLVM 14.

compiler/rustc_span/src/symbol.rs

@@ -1202,6 +1202,7 @@ symbols! {
         rustc,
         rustc_allocator,
         rustc_allocator_nounwind,
+        rustc_allocator_zeroed,
         rustc_allow_const_fn_unstable,
         rustc_allow_incoherent_impl,
         rustc_allowed_through_unstable_modules,
@@ -1214,6 +1215,7 @@ symbols! {
         rustc_const_stable,
         rustc_const_unstable,
         rustc_conversion_suggestion,
+        rustc_deallocator,
         rustc_def_path,
         rustc_diagnostic_item,
         rustc_diagnostic_macros,
@@ -1258,6 +1260,7 @@ symbols! {
         rustc_private,
         rustc_proc_macro_decls,
         rustc_promotable,
+        rustc_reallocator,
         rustc_regions,
         rustc_reservation_impl,
         rustc_serialize,