[pseudo] Allow mixed designated/undesignated init lists.

This isn't allowed by the standard grammar but is allowed in C, and clang/GCC
permit it as an extension.
It avoids the need to determine which type of list we have in error-recovery.

While here, also support array index designators `{ [4]=1 }` which are
also legal in C, and common extensions in C++.

Differential Revision: https://reviews.llvm.org/D128687