`Sema::getCurFunctionDecl(AllowLambda = false)` returns a nullptr when
the lambda declaration is outside a function (for example, when
assigning a lambda to a static constexpr variable).
This triggered an assertion in
`SemaAMDGPU::CheckAMDGCNBuiltinFunctionCall`.
Using `Sema::getCurFunctionDecl(AllowLambda = true)` returns the
declaration of the enclosing lambda.
Stumbled with this issue when refactoring some code in CK.
d995b2ebdc