Shilei Tian f3a674a2ef [RFC][Clang][AMDGPU] Emit only delta target-features to reduce IR bloat (#176533) ...

Currently, AMDGPU functions have `target-features` attribute populated
with all default features for the target GPU. This is redundant because
the backend can derive these defaults from the `target-cpu` attribute
via `AMDGPUTargetMachine::getFeatureString()`.

In this PR, for AMDGPU targets only:

- Functions without explicit target attributes no longer emit
`target-features`
- Functions with `__attribute__((target(...)))` or `-target-feature`
emit only features that differ from the target's defaults (delta)

The backend already handles missing `target-features` correctly by
falling back to the TargetMachine's defaults.

A new cc1 flag `-famdgpu-emit-full-target-features` is added to emit
full features when needed.

Example:

Before:

```llvm
attributes #0 = { "target-cpu"="gfx90a" "target-features"="+16-bit-insts,+atomic-buffer-global-pk-add-f16-insts,+atomic-fadd-rtn-insts,+ci-insts,+dl-insts,+dot1-insts,+dot2-insts,..." }
```

After (default):

```llvm
attributes #0 = { "target-cpu"="gfx90a" }
```

After (with explicit `+wavefrontsize32` override):

```llvm
attributes #0 = { "target-cpu"="gfx90a" "target-features"="+wavefrontsize32" }
```

2026-01-20 14:49:35 -05:00

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