[SR-15982] SymbolKit crashes when encoding unknown mixins

[franklinsch]

Previous ID	SR-15982
Radar	rdar://problem/90348996
Original Reporter	@franklinsch
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Swift-DocC
Labels	Improvement
Assignee	None
Priority	Medium

md5: 7a780d28224eef5a62504852074094c5

Issue Description:

SymbolKit decodes unknown Symbol mixins just fine by adding them to Symbol.mixins, but {{fatalError}}s when encoding unknown mixins: https://github.com/apple/swift-docc-symbolkit/blob/main/Sources/SymbolKit/SymbolGraph/Symbol/Symbol.swift#L180.

For example, when decoding a symbol graph from the Swift compiler (which includes the location mixin) and encoding it back, SymbolKit crashes.

We should make SymbolKit encode unknown mixins.

[franklinsch]

@swift-ci create

[theMomax]

This issue was solved by swiftlang/swift-docc-symbolkit#39.

[Kyle-Ye]

apple/swift-docc-symbolfix#39 fix the issue in the upstream.
#369 update the SymbolKit dependency commit in swift-docc.