[clang] Array list initialization (pre-p0388)

Extends handling of list initialization of bounded array parameters.
This adds the missing checks on converting each initializer for both
std::initializer_list and arrays. And extends
CompareImplicitConversionSequence to compares array size, for two
conversions to array type.

As noted in this patch, there's a defect in the std concerning the
partial orderability of conversion sequences.  DR2492 has a suggested
direction that will be simple to add once it (hopefully) is accepted.

Differential Revision: https://reviews.llvm.org/D103088