These represent types which don't store any interesting data for us to
measure, but which are required by a real container so can not be
replaced with our own generated class types.
std::allocator often has bad DWARF, so it must be replaced after the
DWARF is fixed up in Flattener. The others could be replaced earlier in
the transformation process if desired, but I've left them all together
for simplicity for now.
This fixes the folly::fbstring tests.
