[clangd] Fix broken assertion

Summary:
This assertion was bad. It will show up once we start running preamble
thread async. Think about the following case:

- Update 1
    builds a preamble, and an AST. Caches the AST.
- Update 2
    Invalidates the cache, preamble hasn't changed.
- Update 3
    Invalidates the cache, preamble hasn't changed
- Read
    builds AST using preamble v1, and caches it.
    preamble for v2 gets build, cache isn't invalidated since preamble is same.
    generateDiags tries to reuse cached AST but latest version is 3 not 2, so
    assertion fails.

Reviewers: sammccall

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77664