[clang] Treat function parameter scope as an immediate function context

This results in expressions that appear in default function argument not
being checked for being actual constant expressions.
This aligns clang's behavior with the standard and fixes one of the
examples from https://wg21.link/P1073R3.

Reviewed By: shafik, cor3ntin

Differential Revision: https://reviews.llvm.org/D145251