Explicit CopyString, remove (const char*) constructors.

This gets rid of the (litstr) StringData and StackStringData
constructors, but keeps String(litstr).  Also rename all
the instances of AttachLiteral to CopyString, since they now
mean the same thing.

hphp/runtime/base/string_util.cpp

@@ -301,7 +301,7 @@ Variant StringUtil::ChunkSplit(CStrRef body, int chunklen /* = 76 */,
 String StringUtil::CEncode(CStrRef input, CStrRef charlist) {
   String chars = charlist;
   if (chars.isNull()) {
-    chars = String("\\\x00\x01..\x1f\x7f..\xff", 10, AttachLiteral);
+    chars = String("\\\x00\x01..\x1f\x7f..\xff", 10, CopyString);
   }
   if (input.empty() || chars.empty()) return input;
   int len = input.size();