Comment AST: Recognize function-like objects via return type (NFC)

Instead of pretending that function pointer type aliases or variables
are functions, and thereby losing the information that they are type
aliases or variables, respectively, we use the existence of a return
type in the DeclInfo to signify a "function-like" object.

That seems pretty natural, since it's also the return type (or parameter
list) from the DeclInfo that we compare the documentation with.

Addresses a concern voiced in D111264#3115104.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D113691