[Syntax] Add a helper to find expansion by its first spelled token

Summary: Used in clangd for a code tweak that expands a macro.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62954

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@363698 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed