comgr: upstream AMDGPU note-owner / ISA-version-note constants to public LLVM headers

[harsh-amd]

Context

While preparing the HotSwap infrastructure PRs (#2201, #2202, #2203) we had to
locally redefine two AMDGPU-specific ELF note constants because they only live
in target-internal LLVM headers that are not exported by a comgr standalone
install, and are not currently registered in any public llvm::ELF enum.

Constants that need a public home

1. AMDGPU note owner string ("AMDGPU")

Currently defined in llvm/lib/Target/AMDGPU/AMDGPUPTNote.h:

https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/AMDGPU/AMDGPUPTNote.h#L22-L29

This is a target-internal header (under llvm/lib/Target/AMDGPU/) and is not
installed or usable by out-of-tree consumers such as Comgr in its standalone
build mode.

Comgr's hotswap PRs duplicate this constant as:

// amd/comgr/src/comgr-hotswap-internal.h
static constexpr llvm::StringLiteral kAmdgpuNoteOwner = \"AMDGPU\";

2. AMDGPU ISA-version note type (NT_AMDGPU_ISA_VERSION = 27)

There is no corresponding entry in llvm/include/llvm/BinaryFormat/ELF.h for
this note type. The note-name block of that header registers many
vendor-specific NT_ constants (see for example the AMD GPU block at
https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/BinaryFormat/ELF.h#L1980-L1995),
but not the ISA-version note used by AMDGPU code-object v3+.

Comgr's hotswap PRs duplicate the raw value as:

// amd/comgr/src/comgr-hotswap-internal.h
static constexpr uint32_t kNoteTypeIsaVersion = 27;

Proposal

Upstream both constants into public LLVM headers so Comgr (and any other
out-of-tree consumer) can use the canonical LLVM definitions without duplication
or divergence risk:

1. Move or re-export AMDGPU::ElfNote::NoteNameV3 (and any related
   NoteName* constants) from AMDGPUPTNote.h into a public header such as
   llvm/include/llvm/BinaryFormat/AMDGPUMetadataVerifier.h or a new
   llvm/include/llvm/BinaryFormat/AMDGPU.h.
2. Add the AMDGPU ISA-version note type (value 27) to the AMDGPU block in
   llvm/include/llvm/BinaryFormat/ELF.h, e.g. as NT_AMDGPU_ISA_VERSION.

Once the public definitions exist, Comgr will switch to them and drop the
local duplicates.

Context PRs

Tracked in reviewer feedback on:

• [AMDGPU] comgr: add HotSwap ELF infrastructure (1/3) #2201 comgr: add HotSwap ELF infrastructure (1/3)
  • Thread on kAmdgpuNoteOwner: [AMDGPU] comgr: add HotSwap ELF infrastructure (1/3) #2201 (comment)
  • Thread on note type 27: [AMDGPU] comgr: add HotSwap ELF infrastructure (1/3) #2201 (comment)