[stdlib] Used storeRaw for constrained storeBytes.

Implement the overload of storeBytes where value is constrained to
BitwiseCopyable via Builtin.storeRaw.

Author: nate-chandler

stdlib/public/core/UnsafeRawPointer.swift

@@ -1316,12 +1316,7 @@ public struct UnsafeMutableRawPointer: _Pointer {
   public func storeBytes<T : _BitwiseCopyable>(
     of value: T, toByteOffset offset: Int = 0, as type: T.Type
   ) {
-    withUnsafePointer(to: value) { source in
-      _memcpy(
-        dest: (self + offset), 
-        src: source, 
-        size: UInt(MemoryLayout<T>.size))
-    }
+    Builtin.storeRaw(value, (self + offset)._rawValue)
   }
 #endif
   @inlinable