logd: fix various clang-tidy issues

In order of severity:
1) Add a CHECK() that a pointer is not nullptr, where the analyzer
   believes this is possible.
2) Add `final` appropriately to functions called from constructors.
3) Add missing cloexec flags.
4) Add missing `noexcept` and other subtle performance warnings

Test: build with clang-tidy
Change-Id: Ifd9a1299a51027a47382926b2224748b5750d6cf