Fixes #157492

On z/OS `strnlen()` is not available by default so we use the wrapper
header `zOSSupport.h` to make it visible. This will fix the following
error:

```
clang/lib/AST/ByteCode/Context.cpp:250:16: error: use of undeclared identifier 'strnlen'
  250 |       Result = strnlen(reinterpret_cast<const char *>(Ptr.getRawAddress()), N);
      |                ^~~~~~~
1 error generated.
```

Fixes #153948

Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>

Fix two older FIXME items from the `functions.cpp` test.

So the static invoker's Function still points to the static invoker
instead of the call operator of the lambda record. This is important for
a later commit.

We just called `getInit()`, which isn't always correct and used the
wrong initializer in the module test case.

Remove support for saving a `Pointer` in the `EvaluationResult` since
that was unused. Add `stealAPValue()` which moves the `APValue` out of
the `EvaluationResult` to avoid a copy at the end of the evaluation.

And make use of those.

These changes are split from prior PR #155028, in order to decrease the
size of that PR and facilitate review.

This has been a long-standing problem, but we didn't use to call the
destructors of items on the stack unless we explicitly `pop()` or
`discard()` them.

When interpretation was interrupted midway-through (because something
failed), we left `Pointer`s on the stack. Since all `Block`s track what
`Pointer`s point to them (via a doubly-linked list in the `Pointer`),
that meant we potentially leave deallocated pointers in that list. We
used to work around this by removing the `Pointer` from the list before
deallocating the block.

However, we now want to track pointers to global blocks as well, which
poses a problem since the blocks are never deallocated and thus those
pointers are always left dangling.

I've tried a few different approaches to fixing this but in the end I
just gave up on the idea of never knowing what items are in the stack.
We already have an `ItemTypes` vector that we use for debugging
assertions. This patch simply enables this vector unconditionally and
uses it in the abort case to properly `discard()` all elements from the
stack. That's a little sad IMO but I don't know of another way of
solving this problem.

As expected, this is a slight hit to compile times:
https://llvm-compile-time-tracker.com/compare.php?from=574d0a92060bf4808776b7a0239ffe91a092b15d&to=0317105f559093cfb909bfb01857a6b837991940&stat=instructions:u