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

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guillem-bartrina-sonarsource wrote:

Changing when some decls are linked in the target AST is a delicate change with a large blast radius. This patch has been tested by performing CTU analysis on a large set of open-source projects, and no issues were observed.

There was a unit test exercising a path where an error occurs between creation and linking, but I do not know the original reasoning behind it. Nothing seems to break.

Let me know if you have any concerns.

CC: @martong

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