Remove "illegal" UnsafePointer casts from the stdlib.

Update for SE-0107: UnsafeRawPointer

This adds a "mutating" initialize to UnsafePointer to make
Immutable -> Mutable conversions explicit.

These are quick fixes to stdlib, overlays, and test cases that are necessary
in order to remove arbitrary UnsafePointer conversions.

Many cases can be expressed better up by reworking the surrounding
code, but we first need a working starting point.

stdlib/public/core/UnsafeRawPointer.swift.gyb

@@ -27,13 +27,21 @@ public struct Unsafe${Mutable}RawPointer : Strideable, Hashable, _Pointer {
   /// Implements conformance to the public protocol `_Pointer`.
   public let _rawValue: Builtin.RawPointer
 
-  // Construct ${a_Self} from another ${a_Self}.
-  // FIXME: Why is this necessary?
+  /// Construct ${a_Self} from another ${a_Self}.
   @_transparent
   public init(_ other: Unsafe${Mutable}RawPointer) {
     self = other
   }
 
+  /// Construct ${a_Self} from another ${a_Self}.
+  ///
+  /// Returns nil if `other` is nil.
+  @_transparent
+  public init?(_ other: Unsafe${Mutable}RawPointer?) {
+    guard let unwrapped = other else { return nil }
+    self = unwrapped
+  }
+
   /// Convert a builtin raw pointer to ${a_Self}.
   @_transparent
   public init(_ _rawValue: Builtin.RawPointer) {