Fix prvalue to xvalue conversion address space assignment

[Naghasan]

The common address deduction rules assumed prvalue-to-xvalue conversion can only happens inside a function.
The patch aligns the rules to match declaration rules.

It is a bit of a corner case, but the spec treated all 3 conversions bellow in the same way.

constexpr const int& foo = 5;

class Foo {
public:
  static constexpr const int& foo = 42;
};

void bar() {
    const int& foo = 42;
}

[CLAassistant]

All committers have signed the CLA.

[keryell]

@Naghasan ping!

[Naghasan]

@keryell I extended the initial text to also account for the context in which prvalue to xvalue is taking place. I find it a bit "hand wavy" but still acceptable.

The new commit also account for this:

void bar() {
    static constexpr const int& foo = 42;
}

[keryell]

Thanks!

[Naghasan]

Resolved conflict: rebased to head and ran reflow