Apply some clang-tidy fixes.

This patch was generated using clang-tidy:

  clang-tidy -checks="-*,..." -fix

The following set of checks was enabled:

  modernize-use-nullptr,modernize-deprecated-headers,modernize-make-shared,
  modernize-redundant-void-arg,modernize-use-bool-literals,
  modernize-use-emplace,modernize-use-equals-default,
  google-readability-namespace-comments,misc-unused-using-decls

Add NOLINT to have clang-tidy (among other tools) ignore some lines
that are not meant to be changed.