[clang] Fix crash 'Cannot get layout of forward declarations' during CTU static analysis (#156056)
When a type is imported with `ASTImporter`, the "original declaration"
of the type is imported. In some cases this is not the definition
(of the class). Before the fix the definition was only imported if
there was an other reference to it in the AST to import. This is not
always the case (like in the added test case), if not the definition
was missing in the "To" AST which can cause the assertion later.
llvm-project
47de90e2 - [clang] Fix crash 'Cannot get layout of forward declarations' during CTU static analysis (#156056)
