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_middle/src/middle/codegen_fn_attrs.rs

@@ -50,7 +50,7 @@ bitflags! {
         /// the hot path.
         const COLD                      = 1 << 0;
         /// `#[rustc_allocator]`: a hint to LLVM that the pointer returned from this
-        /// function is never null.
+        /// function is never null and the function has no side effects other than allocating.
         const ALLOCATOR                 = 1 << 1;
         /// An indicator that function will never unwind. Will become obsolete
         /// once C-unwind is fully stabilized.
@@ -91,6 +91,12 @@ bitflags! {
         const NO_COVERAGE               = 1 << 15;
         /// `#[used(linker)]`: indicates that LLVM nor the linker can eliminate this function.
         const USED_LINKER               = 1 << 16;
+        /// `#[rustc_deallocator]`: a hint to LLVM that the function only deallocates memory.
+        const DEALLOCATOR               = 1 << 17;
+        /// `#[rustc_reallocator]`: a hint to LLVM that the function only reallocates memory.
+        const REALLOCATOR               = 1 << 18;
+        /// `#[rustc_allocator_zeroed]`: a hint to LLVM that the function only allocates zeroed memory.
+        const ALLOCATOR_ZEROED          = 1 << 19;
     }
 }