[stdlib] underscore APIs made visible by r22212

That commit made a protocol public.  The protocol was underscored but
some of its APIs were not, and those became unintentionally publicly
visible.  This commit corrects that problem.  Since Builtin.RawPointer
properties were being renamed from "value" to "_rawValue" for clarity,
COpaquePointer got that treatment too, even though it wasn't strictly
necessary, for consistency.

Swift SVN r22252

stdlib/core/StaticString.swift

@@ -122,7 +122,7 @@ public struct StaticString
     unicodeScalar: Builtin.Int32
   ) {
     self._startPtrOrData =
-      unsafeBitCast(UWord(UInt32(unicodeScalar)), COpaquePointer.self).value
+      unsafeBitCast(UWord(UInt32(unicodeScalar)), COpaquePointer.self)._rawValue
     self._byteSize = 0.value
     self._flags = 0.value
   }