This fixes a crash at runtime when destroying a Swift array of values of a C++ foreign reference type.
Swift optimizes the amount of metadata emitted for `_ContiguousArrayStorage<Element>` by reusing `_ContiguousArrayStorage<AnyObject>` whenever possible (see `getContiguousArrayStorageType`). However, C++ foreign reference types are not `AnyObject`s, since they have custom retain/release operations.
This change disables the `_ContiguousArrayStorage` metadata optimization for C++ reference types, which makes sure that `swift_arrayDestroy` will call the correct release operation for elements of `[MyCxxRefType]`.
rdar://127154770
This teaches IRGen to only emit a lifetime operation (retain or release) for a C++ foreign reference type if the pointer is not `nullptr`.
Previously the compiler would in some cases emit a release call for `nullptr`, which breaks the assumption that the argument to a custom release function is `_Nonnull`. For instance:
```
var globalOptional: MyRefType? = nil
func foo() { globalOptional = MyRefType.create() }
```
When emitting IR for the assignment operation to `globalOptional`, the compiler would emit code to first retrieve the existing value of `globalOptional` and release it. If the value is `nil`, it does not need to be released.
rdar://97532642
