[clang] [clang][ASTImporter] Link FunctionDecl into its DeclContext before importing its body (PR #214009)

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guillem-bartrina-sonarsource wrote:

> The ASTImporter is a separate component. You should test it with their testing infra - which is unittests.

I'm not sure I understand your point. In this PR, I am adding a new unit test and a lit test to demonstrate the issue, and I am amending an existing unit test to cover the new behavior. The testing on a large set of open-source projects is strictly on top of these tests. What exactly am I missing?

https://github.com/llvm/llvm-project/pull/214009