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

[sammccall]

Summary

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.

[nridge]

Nice!

clang-tools-extra/clangd/unittests/InlayHintTests.cpp

539

Maybe add a comment like "use digraphs to place a literal ] in an annotation range"

[Trass3r]

clang-tools-extra/clangd/unittests/InlayHintTests.cpp

626

Should there be a test for void too or does it just work?

[sammccall]

clang-tools-extra/clangd/unittests/InlayHintTests.cpp

626

Added a test verifying that an auto-typed function/lambda with no return statements gets a -> void hint.