[clang] [clang] Fix of a crash 'Cannot get layout of forward declarations!' during CTU static analysis (PR #155375)

[Balázs Kéri via cfe-commits]

balazske wrote:

It is possible to set the `DeclForType` to the definition (if one exists) and import only this value. This would be only one import. The only issue is that later the value is used in `getTagType` instead of the original value (corresponding to imported `getOriginalDecl`) and I am not sure if this is entirely correct (but all tests pass in this case too).

https://github.com/llvm/llvm-project/pull/155375