[clangd] Function return type hints: support lambdas, don't duplicate "->"

While here, fix an ugliness:
  auto foo()->auto { return 42; }
This (silly) code gains a "-> int" hint. While correct and useful, it renders as
  auto foo()->int->auto { return 42; }
which is confusing enough to do more harm than good I think.

Differential Revision: https://reviews.llvm.org/D120416