Recommit [YAML IO] Check that mapping doesn't contain duplicating keys

The revert reason is fixed in D143727 (test changes).

According to YAML specification keys must be unique for a mapping node:
"The content of a mapping node is an unordered set of key/value node pairs, with
the restriction that each of the keys is unique".

Differential Revision: https://reviews.llvm.org/D140474