C++ PATCH for c++/55922 and 63151 (list-value-initialization of base)

[Jason Merrill]

Using build_value_init in a base initialization is wrong, because it
calls the complete object constructor and misses protected access.  So
let's handle list-value-initialization in expand_aggr_init_1.

Tested x86_64-pc-linux-gnu, applying to trunk.