[clang-tidy] Correctly handle evaluation order of designated initializers.

As designated initializers show up only in the syntactic form of the
InitListExpr, we need to make sure we're searching both forms of the
InitListExpr when determining successors in the evaluation order.

This fixes a bug in bugprone-use-after-move where previously we erroneously
concluded that two designated initializers were unsequenced. The newly added
tests fail without the fix.

Differential Revision: https://reviews.llvm.org/D145906