Diagnose an unused result from a call through a function pointer whose return...

Diagnose an unused result from a call through a function pointer whose return type is marked [[nodiscard]].

When a function returns a type and that type was declared [[nodiscard]], we diagnose any unused results from that call as though the function were marked nodiscard. The same behavior should apply to calls through a function pointer.

This addresses PR31526.

llvm-svn: 350317