Fix false positive related to handling of [[noreturn]] function pointers

Before this change, the `NoReturnFunctionChecker` was missing function pointers
with a `[[noreturn]]` attribute, while `CFG` was constructed taking that into
account, which leads CSA to take impossible paths. The reason was that the
`NoReturnFunctionChecker` was looking for the attribute in the type of the
entire call expression rather than the type of the function being called.

This change makes the `[[noreturn]]` attribute of a function pointer visible
to `NoReturnFunctionChecker`. This leads to a more coherent behavior of the
CSA on the AST involving.

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D135682