[Syntax] Fix macro-arg handling in TokenBuffer::spelledForExpanded

A few cases were not handled correctly. Notably:
  #define ID(X) X
  #define HIDE a ID(b)
  HIDE
spelledForExpanded() would claim HIDE is an equivalent range of the 'b' it
contains, despite the fact that HIDE also covers 'a'.

While trying to fix this bug, I found findCommonRangeForMacroArgs hard
to understand (both the implementation and how it's used in spelledForExpanded).
It relies on details of the SourceLocation graph that are IMO fairly obscure.
So I've added/revised quite a lot of comments and made some naming tweaks.

Fixes https://github.com/clangd/clangd/issues/1289

Differential Revision: https://reviews.llvm.org/D134618