This is a split of https://github.com/llvm/llvm-project/pull/125756

When a test depends on a new debugserver feature/fix, the API test must
be marked @skipIfOutOfTreeDebugserver because the macOS CI bots test
using the latest Xcode release debugserver. But over time all of these
fixes & new features are picked up in the Xcode debugserver and these
skips can be removed.

We may see unexpected test failures from removing all of these 1+ year
old skips, but that's likely a separate reason the test is failing that
is being papered over by this skip.

For 0d vector type the rewrite crashes.

Implement HLSLElementwiseCast excluding support for splat cases
Do not support casting types that contain bitfields.
Partly closes #100609 and partly closes #100619

Summary:
Currently the `-Xarch` argument needs to re-parse the option, which goes
through every single registered argument. This causes errors when trying
to pass `-O1` through it because it thinks it's a DXC option. This patch
changes the behavior to only allow `clang` options. Concievably we could
detect the driver mode to make this more robust, but I don't know if
there are other users for this.

Fixes: https://github.com/llvm/llvm-project/issues/110325

Summary:
This patch cleans up how we query the offloading toolchain. We create a
single that is more similar to the existing `getToolChain` driver
function and make all the offloading handlers use it.

The DAG will now receive a callback whenever a new instruction is
created and will update itself accordingly.