The patch updated with a test case is attached.

gdb/ChangeLog:

        * gnu-v3-abi.c (gnuv3_pass_by_reference): Call TYPE_TARGET_TYPE
        on the arg type of a constructor only if it is of reference type.

gdb/testsuite/ChangeLog:

        * gdb.cp/non-trivial-retval.cc: Add a test case.
        * gdb.cp/non-trivial-retval.exp: Add a test.