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.

test/1_stdlib/NSStringAPI.swift

@@ -204,7 +204,9 @@ NSStringAPIs.test("init(utf8String:)") {
     i += 1
   }
   up[i] = 0
-  expectOptionalEqual(s, String(utf8String: UnsafePointer(up)))
+  let cstr = UnsafeMutableRawPointer(up)
+    .bindMemory(to: CChar.self, capacity: 100)
+  expectOptionalEqual(s, String(utf8String: cstr))
   up.deallocate(capacity: 100)
 }