[clang][ASTImporter] New fix for default template parameter values. (#101836)

Commit e4440b8 added a change that introduced new crash in an
incorrectly handled case. This is fixed here. Default argument
definition or inheritance is preserved in the "To" AST compared to
the "From". If the default argument is defined already in the "To"
AST it can be duplicated at the import.

GitOrigin-RevId: 9c72a308d839a27ffcbb0c67104baceb1871c50e