[pseudo] Eliminate multiple-specified-types ambiguities using guards

Motivating case: `foo bar;` is not a declaration of nothing with `foo` and `bar`
both types.

This is a common and critical ambiguity, clangd/AST.cpp has 20% fewer
ambiguous nodes (1674->1332) after this change.

Differential Revision: https://reviews.llvm.org/D130337