- When it's safe, ActionResult uses the low bit of the pointer for
  the "invalid" flag rather than a separate "bool" value. This keeps
  GCC from generating some truly awful code, for a > 3x speedup in the
  result-passing microbenchmark.
  - When DISABLE_SMART_POINTERS is defined, store an ActionResult
  within ASTOwningResult rather than an ASTOwningPtr. Brings the
  performance benefits of the above to smart pointers with
  DISABLE_SMART_POINTERS defined.

Sadly, these micro-benchmark performance improvements don't seem to
make much of a difference on Cocoa.h right now. However, they're
harmless and might help with future optimizations.

llvm-svn: 63061