[clang][ASTImporter] Fix of possible crash "Did not find base!". (#67680)

A problem with AST import could lead to multiple instances of the same
template class specialization, with different template arguments. The
difference was caused by pointers to different declarations of the same
function.
Problem is fixed by using the canonical declaration at import.

Co-authored-by: Balázs Kéri <balazs.keri@ericsson.com>