DXMT: add LLVM build compatibility fixes

[moreaki]

Summary

Add minimal DXMT/LLVM compatibility fixes. These are independent from the earlier GUI repaint and HiDPI work.

The VBoxDxMt link now derives LLVM libraries from the configured llvm-config when available. That avoids hard-coding a Homebrew libLLVM-N.dylib name and supports shared LLVM packages, while preserving the original component-library list as a fallback for Oracle/devtools trees without llvm-config.

The AIR argmemonly attribute handling is version-gated: LLVM 15 and older use the original enum attribute path, while LLVM 16 and newer use the MemoryEffects API required by LLVM 17+.

The AIR/DXBC conversion code now passes explicit element types into GEP construction instead of relying on typed-pointer introspection. For LLVM 15 typed-pointer builds, the vector-array helper preserves the aggregate element type where needed so IRBuilder::CreateGEP does not assert on mismatched source element types. The shuffle-mask sentinel is also kept local so the code is not tied to an LLVM-version-specific mask constant name.

Why

The pointer/API issues are real DXMT/LLVM compatibility bugs:

• getNonOpaquePointerElementType() is invalid once LLVM opaque pointers are in use.
• The GEP crash under LLVM 15 is also real: the IR builder was being given the wrong element type for typed-pointer aggregate accesses.
• ArgMemOnly versus MemoryEffects::argMemOnly() is LLVM API drift.
• The shuffle-mask sentinel naming is LLVM-version drift.

Compatibility note

This patch intentionally does not hard-code libLLVM-15.dylib. The source should build against LLVM 15 and newer LLVM APIs where possible, while keeping the link flexible through llvm-config and preserving the static component-library fallback.

On the macOS/Arm test host, the installed VirtualBox 7.2.12 VBoxDxMt.dylib does not have a dynamic libLLVM-17.dylib dependency according to otool -L. It contains LLVM symbols directly, and strings reports llvm-mc (based on LLVM 15.0.7). That suggests the shipped binary may still be produced through an LLVM 15-era static component-library toolchain.

For the macOS/Arm Metal/AIR path, LLVM 15.0.7 is the currently verified producer. Local LLVM 17/22 builds can get further through the source build, but newer upstream LLVM bitcode may not be accepted by Apple's Metal compiler without a downgrade/normalization layer or a different shader handoff boundary; this showed up locally as Failed to upgrade function bitcode.

Validation

• Applied independently to origin/main.
• git diff --check passes.
• Configured a clean PR worktree on macOS/Arm with kBuild initialized.
• Built VBoxDxMt successfully with Homebrew LLVM 15.0.7:
  • VBOX_LLVM_PATH=/opt/homebrew/opt/llvm@15
  • VBOX_LLVM_CONFIG=/opt/homebrew/opt/llvm@15/bin/llvm-config
• otool -L out/darwin.arm64/release/dist/VirtualBox.app/Contents/MacOS/VBoxDxMt.dylib shows /opt/homebrew/opt/llvm@15/lib/libLLVM.dylib with current version 15.0.7.

Refs #742.

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