[-Wunsafe-buffer-usage] Add AST info to the unclaimed DRE debug notes for analysis

- For a better understand of what the unsupported cases are, we add
  more information to the debug note---a string of ancestor AST nodes
  of the unclaimed DRE. For example, an unclaimed DRE p in an
  expression `*(p++)` will result in a string starting with
  `DRE ==> UnaryOperator(++) ==> Paren ==> UnaryOperator(*)`.

- To find out the most common patterns of those unsupported use cases,
  we add a simple script to build a prefix tree over those strings and
  count each prefix. The script reads input line by line, assumes a
  line is a list of words separated by `==>`s, and builds a prefix tree
  over those lists.

Reviewed by: t-rasmud (Rashmi Mudduluru), NoQ (Artem Dergachev)

Differential revision: https://reviews.llvm.org/D158561