[clangd] Introduce ASTHooks to FeatureModules

These can be invoked at different stages while building an AST to let
FeatureModules implement features on top of it. The patch also
introduces a sawDiagnostic hook, which can mutate the final clangd::Diag
while reading a clang::Diagnostic.

Differential Revision: https://reviews.llvm.org/D98499