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.

src/test/codegen/vec-calloc.rs

@@ -1,6 +1,7 @@
 // compile-flags: -O
 // only-x86_64
 // ignore-debug
+// min-llvm-version: 15.0
 
 #![crate_type = "lib"]
 
@@ -142,3 +143,8 @@ pub fn vec_non_zero_tuple(n: usize) -> Vec<(i16, u8, char)> {
     // CHECK: ret void
     vec![(0, 0, 'A'); n]
 }
+
+// Ensure that __rust_alloc_zeroed gets the right attributes for LLVM to optimize it away.
+// CHECK: declare noalias ptr @__rust_alloc_zeroed(i64, i64 allocalign) unnamed_addr [[RUST_ALLOC_ZEROED_ATTRS:#[0-9]+]]
+
+// CHECK-DAG: attributes [[RUST_ALLOC_ZEROED_ATTRS]] = { {{.*}} allockind("alloc,zeroed,aligned") allocsize(0) uwtable "alloc-family"="__rust_alloc" {{.*}} }