- _Float16 is now accepted by Clang.

- The half IR type is fully handled by the backend.

- These values are passed in FP registers and converted to/from float around
  each operation.

- Compiler-rt conversion functions are now built for s390x including the missing
  extendhfdf2 which was added.

Fixes #50374

The recently announced IBM z17 processor implements the architecture
already supported as "arch15" in LLVM. This patch adds support for "z17"
as an alternate architecture name for arch15.

This patch also add the scheduler description for the z17 processor,
provided by Jonas Paulsson.

This is straightforward as we already had all the necessary
instructions, they simply were not wired up.

Also allows implementing the vec_round intrinsic via the
standard llvm.roundeven IR instead of a platform intrinsic now.

This PR removes the old `nocapture` attribute, replacing it with the new
`captures` attribute introduced in #116990. This change is
intended to be essentially NFC, replacing existing uses of `nocapture`
with `captures(none)` without adding any new analysis capabilities.
Making use of non-`none` values is left for a followup.

Some notes:
* `nocapture` will be upgraded to `captures(none)` by the bitcode
   reader.
* `nocapture` will also be upgraded by the textual IR reader. This is to
   make it easier to use old IR files and somewhat reduce the test churn in
   this PR.
* Helper APIs like `doesNotCapture()` will check for `captures(none)`.
* MLIR import will convert `captures(none)` into an `llvm.nocapture`
   attribute. The representation in the LLVM IR dialect should be updated
   separately.

This patch adds support for the next-generation arch15
CPU architecture to the SystemZ backend.

This includes:
- Basic support for the new processor and its features.
- Detection of arch15 as host processor.
- Assembler/disassembler support for new instructions.
- Exploitation of new instructions for code generation.
- New vector (signed|unsigned|bool) __int128 data types.
- New LLVM intrinsics for certain new instructions.
- Support for low-level builtins mapped to new LLVM intrinsics.
- New high-level intrinsics in vecintrin.h.
- Indicate support by defining  __VEC__ == 10305.

Note: No currently available Z system supports the arch15
architecture.  Once new systems become available, the
official system name will be added as supported -march name.

In preparation of making `-Wreturn-type` default to an error (as there
is virtually no situation where you’d *want* to fall off the end of a
function that is supposed to return a value), this patch fixes tests
that have relied on this being only a warning, of which there seem 
to be 3 kinds:

1. Tests which for no apparent reason have a function that triggers the
warning.

I suspect that a lot of these were on accident (or from before the
warning was introduced), since a lot of people will open issues w/ their
problematic code in the `main` function (which is the one case where you
don’t need to return from a non-void function, after all...), which
someone will then copy, possibly into a namespace, possibly renaming it,
the end result of that being that you end up w/ something that
definitely is not `main` anymore, but which still is declared as
returning `int`, and which still has no return statement (another reason
why I think this might apply to a lot of these is because usually the
actual return type of such problematic functions is quite literally
`int`).
  
A lot of these are really old tests that don’t use `-verify`, which is
why no-one noticed or had to care about the extra warning that was
already being emitted by them until now.

2. Tests which test either `-Wreturn-type`, `[[noreturn]]`, or what
codegen and sanitisers do whenever you do fall off the end of a
function.

3. Tests where I struggle to figure out what is even being tested
(usually because they’re Objective-C tests, and I don’t know
Objective-C), whether falling off the end of a function matters in the
first place, and tests where actually spelling out an expression to
return would be rather cumbersome (e.g. matrix types currently don’t
support list initialisation, so I can’t write e.g. `return {}`).

For tests that fall into categories 2 and 3, I just added
`-Wno-error=return-type` to the `RUN` lines and called it a day. This
was especially necessary for the former since `-Wreturn-type` is an
analysis-based warning, meaning that it is currently impossible to test
for more than one occurrence of it in the same compilation if it
defaults to an error since the analysis pass is skipped for subsequent
functions as soon as an error is emitted.

I’ve also added `-Werror=return-type` to a few tests that I had already
updated as this patch was previously already making the warning an error
by default, but we’ve decided to split that into two patches instead.

BasicAA currently tries to support addrspacecasts that change the index
width by performing the decomposition in the maximum of all index widths
and then trying to fix this up with in-place sign extends to get correct
overflow behavior if the actual index width is smaller.

However, even in the case where we don't mix different index widths and
just have an index width that is smaller than the maximum, the behavior
is incorrect (see test), because we only perform the index width
adjustment during decomposition and not any of the later logic -- and we
don't do anything at all for variable offsets. I'm sure that the case
where we actually mix different index widths is even more broken than
that.

Fix this by not allowing decomposition through index width changes. If
the pointers have different index widths, fall back to a base object
comparison, ignoring the offsets.

This pr includes fixes for original pr##116642.
Implementation for __builtin_setjmp and __builtin_longjmp for SystemZ..

This reverts commit 030bbc92a705758f1131fb29cab5be6d6a27dd1f.

Implementation for __builtin_setjmp and __builtin_longjmp for SystemZ.