This patch is part of a stack that teaches Clang to generate Key Instructions
metadata for C and C++.

RFC:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

The feature is only functional in LLVM if LLVM is built with CMake flag
LLVM_EXPERIMENTAL_KEY_INSTRUCTIONs. Eventually that flag will be removed.

This is an expensive header, only include it where needed. Move some
functions out of line to achieve that.

This reduces time to build clang by ~0.5% in terms of instructions
retired.

The combination of `-fcomplex-arithmetic=promoted` and `mno-x87` for
`double` complex division is leading to a crash.
See https://godbolt.org/z/189G957oY
This patch fixes that.

When `-fcomplex-arithmetic=promoted` is set complex divassign `/=` should
promote to a wider type the same way division (without assignment) does.
Prior to this change, Smith's algorithm would be used for divassign.

Fixes: https://github.com/llvm/llvm-project/issues/131129

Relates-to: https://github.com/llvm/llvm-project/issues/131129

Implement HLSL Aggregate Splat casting that handles splatting for arrays
and structs, and vectors if splatting from a vec1.
Closes #100609 and Closes #100619 
Depends on #118842

Implement HLSLElementwiseCast excluding support for splat cases
Do not support casting types that contain bitfields.
Partly closes https://github.com/llvm/llvm-project/issues/100609 and
partly closes https://github.com/llvm/llvm-project/issues/100619
Re-land #118842 after fixing warning as an error, found by a buildbot.

Reverts llvm/llvm-project#118842

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