[SR-13180] withUnsafePointer should guarantee pointer stability for globals and class properties

[atrick]

Previous ID	SR-13180
Radar	None
Original Reporter	@atrick
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: f898da4d21aaa0b9abd17b57883351ec

Issue Description:

We should provide a guarantee of pointer stability not just within the `withUnsafePointer` closure, but also with across multiple calls with `withUnsafePointer`, `@inout` arguments, and C functions that access the same global.

For example:

C code:

struct MyStruct {
 int field;
};
extern const struct MyStruct myGlobal;
void takePointer(const struct MyStruct *);

Swift code

withUnsafePointer(to: myGlobal) {
 takePointer($0)
}

The implementation should guarantee that the pointer passed to `takePointer` is the address of `myGlobal`:

$0 == &myGlobal

I think this rule should apply to globals and class properties. @lorentey also proposed a "stable storage" language extension, so there might be some overlap with that.