[clang] [clang-tools-extra] Handle recording inheritance for templates (PR #177273)

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timon-ul wrote:

Small update: I haven't had much time to work on this this week, expect more next week, but currently I am pondering why when turning on the `IndexingOptions::IndexImplicitInstantiation` only explicit instantiations get Symbols and if that is desired behaviour. Not sure if this matters at all but for now it leads to a test failure. Also for some odd reason explicit instantiations are treated as implicit ones, so I feel like here is some clean up to do too (see [this call](https://searchfox.org/llvm/source/clang/lib/Index/IndexDecl.cpp#784) returning true on [this](https://searchfox.org/llvm/source/clang/lib/Index/IndexingContext.cpp#189)), which ties again into, what even is the desired behaviour?

https://github.com/llvm/llvm-project/pull/177273