[Syntax] expose API for expansions overlapping a spelled token range.

Summary:
This allows efficiently accessing all expansions (without iterating over each
token and searching), and also identifying tokens within a range that are
affected by the preprocessor (which is how clangd will use it).

Subscribers: ilya-biryukov, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D84009