[Concepts] Add Decls from the outer scope of the current lambda for conversion...

[Concepts] Add Decls from the outer scope of the current lambda for conversion function constraints (#83420)

This fixes the case shown by
https://github.com/llvm/llvm-project/issues/64808#issuecomment-1929131611.

Similar to
https://github.com/llvm/llvm-project/commit/f9caa12328b265b77221fe7a310d4504673d814a

,
we have some calls to constraint checking for a lambda's conversion
function while determining the conversion sequence.

This patch addresses the problem where the requires-expression within
such a lambda references to a Decl outside of the lambda by adding these
Decls to the current instantiation scope.

I'm abusing the flag `ForOverloadResolution` of
CheckFunctionConstraints, which is actually meant to consider the Decls
from parent DeclContexts.

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Co-authored-by: cor3ntin <corentinjabot@gmail.com>