[clang] P2266: apply move elision rules on throw expr nested in function prototypes

Our rules to determine if the throw expression are within the variable
scope were giving a false negative result in case the throw expression
would appear within a decltype in a nested function declaration.

Per P2266R3, the relevant rule is: [expr.prim.id.unqual]/2
```
    if the id-expression (possibly parenthesized) is the operand of a throw-expression, and names an implicitly movable entity that belongs to a scope that does not contain the compound-statement of the innermost lambda-expression, try-block , or function-try-block (if any) whose compound-statement or ctor-initializer encloses the throw-expression.
```

This fixes PR54341.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D127075