Only mark dump() function definitions 'used' in debug builds

This has the dual effect of (1) enabling more dead-stripping in release builds
and (2) ensuring that debug helper functions aren't stripped away in debug
builds, as they're intended to be called from the debugger.

Note that the attribute is applied to definitions rather than declarations in
headers going forward because it's now conditional on NDEBUG:

  /// \brief Mark debug helper function definitions like dump() that should not be
  /// stripped from debug builds.

Requires corresponding macro added in LLVM r198456.