[Syntax] Build mapping from AST to syntax tree nodes

Summary:
Copy of https://reviews.llvm.org/D72446, submitting with Ilya's permission.

Only used to assign roles to child nodes for now. This is more efficient
than doing range-based queries.

In the future, will be exposed in the public API of syntax trees.

Reviewers: gribozavr2

Reviewed By: gribozavr2

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76355