Summary:
- Replace some calls sites from `NDEBUG` to `kIsDebug`.
- Not every call site is changed; there are a few useful cases of
  `NDEBUG` still. `xlog` macros are one of them. Another is where we
  conditionally define a variable in a local scope and then use it in an
  `assert`.
Pull Request resolved: https://github.com/facebook/folly/pull/1128

Reviewed By: aary

Differential Revision: D15490848

Pulled By: yfeldblum

fbshipit-source-id: 4f2d1689f90baa25d3ba9e99a2a388d9cfc43f18