[clangd] Null-check AutoTypeLoc in CollectExtraHighlightings::VisitDeclaratorDecl (#207323)
A Decl's getType() can have a getContainedAutoType() without
its TypeSourceInfo's type having one, because the two types
can differ in type sugar such as DecltypeType which
getContainedAutoType[Loc]() deliberately does not look through.
Fixes https://github.com/llvm/llvm-project/issues/207139