[pseudo] Placeholder disambiguation strategy: always choose second

Mostly mechanics here. Interesting decisions:
 - apply disambiguation in-place instead of copying the forest
   debatable, but even the final tree size is significant
 - split decide/apply into different functions - this allows the hard part
   (decide) to be tested non-destructively and combined with HTML forest easily
 - add non-const accessors to forest to enable apply
 - unit tests but no lit tests: my plan is to test actual C++ disambiguation
   heuristics with lit, generic disambiguation mechanics without the C++ grammar

Differential Revision: https://reviews.llvm.org/D132487