In commit 5651901c54 (Xcode: add support for embedding frameworks,
2020-10-24, v3.20.0-rc1~402^2) we incorrectly reused `PBXBuildFile`
instances when the same framework is embedded in multiple targets,
causing target-specific settings to conflict.
Fixes: #26438
This commit also prepares for embedding things other than
frameworks. In the future, we may want to embed resources and
other types supported by Xcode, so the target properties have
been documented in a way that clearly signals the future intent.
