Clarify and validate generated Objective-C asset symbol support

[onevcat]

This PR supersedes #93 while preserving the original contribution history by continuing from that branch head.

What changed

• keeps the original Objective-C generated asset symbol support from Add support for generated Objective-C asset symbols #93
• adds end-to-end coverage for Objective-C symbol usage in .m files
• documents and tests the current conservative behavior for header-based symbol usage in .h files
• narrows comments to the image-symbol behavior that is actually implemented today
• skips non-member-access file types when collecting generated symbol references

Why

PR #93 fixes a real issue: resources referenced via generated Objective-C asset symbols such as ACImageNameIcFlag were previously reported as unused.

Before merging, two points needed to be made explicit:

1. what behavior we want for header-based symbol exposure
2. that the implementation currently covers image symbols only, not Objective-C color symbols

This follow-up keeps the conservative safety bias for .h files and locks that decision down with tests.

Validation

• swift test --filter GeneratedAssetSymbolTests
• swift test

Notes

• This branch is based directly on the commits from Add support for generated Objective-C asset symbols #93; no cherry-pick was used.
• If this PR is accepted, Add support for generated Objective-C asset symbols #93 can be closed in favor of this one.