Add Compute source build support

[Kyle-Ye]

Summary

• Add Compute source dependency support alongside existing binary xcframework path
• Rename targets Platform → OpenAttributeGraphPlatform and Utilities → OpenAttributeGraphUtilities to avoid name collisions with Compute package
• Default to source build (COMPUTE_BINARY=0), with binary still available via env var
• Add -lswiftDemangle linker setting required by Compute
• Enable Ubuntu CI for Compute source builds (Swift 6.2, libssl-dev, -Xcc -Wno-elaborated-enum-base)
• Simplify OpenAttributeGraphShims product target list

Test plan

• Ubuntu: OPENATTRIBUTEGRAPH_OPENATTRIBUTESHIMS_COMPUTE=1 OPENATTRIBUTEGRAPH_OPENATTRIBUTESHIMS_COMPUTE_BINARY=0 swift build (source)
• macOS/iOS: OPENATTRIBUTEGRAPH_OPENATTRIBUTESHIMS_COMPUTE_BINARY=1 swift build (binary)
• CI workflows validate both paths

[augmentcode]

🤖 Augment PR Summary

Summary: Adds support for building against the Compute package from source in addition to the existing binary XCFramework path.

Changes:

• Introduces a source-vs-binary selection via OPENATTRIBUTEGRAPH_OPENATTRIBUTESHIMS_COMPUTE_BINARY (defaulting to source builds).
• Renames internal Platform/Utilities targets to avoid collisions when Compute is present.
• Adds Compute build settings on OpenAttributeGraphShims, including linking -lswiftDemangle.
• Simplifies the OpenAttributeGraphShims product to only include the shims target.
• Updates CI to enable Ubuntu Compute source builds and explicitly uses binary Compute on macOS/iOS.

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[augmentcode]

Review completed. 1 suggestions posted.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 32.08%. Comparing base (370c3ac) to head (6d0cd14).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #214 +/- ##
===========================================
+ Coverage 17.26% 32.08% +14.82% 
===========================================
Files 72 70 -2 Lines 4883 2596 -2287 ===========================================
- Hits 843 833 -10 + Misses 4040 1763 -2277 

Flag	Coverage Δ
ios	32.10% <ø> (ø)
macos	9.95% <ø> (+6.46%)	⬆️
ubuntu	0.00% <ø> (ø)

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see 30 files with indirect coverage changes

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