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/private/SwiftPrivateLibcExtras/SwiftPrivateLibcExtras.swift

@@ -97,11 +97,17 @@ public func _stdlib_select(
         let readAddr = readfds.baseAddress
         let writeAddr = writefds.baseAddress
         let errorAddr = errorfds.baseAddress
+        func asFdSetPtr(
+          _ p: UnsafeMutablePointer<UInt>?
+        ) -> UnsafeMutablePointer<fd_set>? {
+          return UnsafeMutableRawPointer(p)?
+            .assumingMemoryBound(to: fd_set.self)
+        }
         return select(
           _stdlib_FD_SETSIZE,
-          UnsafeMutablePointer(readAddr),
-          UnsafeMutablePointer(writeAddr),
-          UnsafeMutablePointer(errorAddr),
+          asFdSetPtr(readAddr),
+          asFdSetPtr(writeAddr),
+          asFdSetPtr(errorAddr),
           timeout)
       }
     }