ART: Change UnresolvedMergedType internal representation

Squashed cherry-picks:
* 067f1ed7816cf4eb5d6258ca31b387ddb2073ab7
* 750f7c2827318f6d07620f2ef0321218ea4d8670
* 2f90b3415aadc2587d26c767c6bfb235797119a8
* 2ea7b70b2347969f3735bd0ec1b462bd6d2ff1bd

Bug: 22881413
12 files changed