[clangd] Enable modules to contribute tweaks.

First patch to enable diagnostic fix generation through modules. The
workflow will look like:
- ASTWorker letting modules know about diagnostics while building AST,
modules can read clang::Diagnostic and mutate clangd::Diagnostic through
that hook.
- Modules can implement and expose tweaks to fix diagnostics or act as
general refactorings.
- Tweak::Selection will contain information about the diagnostic
associated with the codeAction request to enable modules to fail their
diagnostic fixing tweakson prepare if need be.

Differential Revision: https://reviews.llvm.org/D98498