Enable more warnings for Clang 6

Enable more warnings for Clang 6

Many useful clang warnings are not enabled with -Wall -Wextra.
Enable all with -Weverything and document which are disabled
for what reason. Only do this for the most recent released
version to avoid having false postives from multiple
versions.

Fix the 9 of the least common issued warnings:
class-varargs range-loop-analysis
unused-template implicit-fallthrough non-virtual-dtor
zero-as-null-pointer-constant
header-hygiene unused-member-function missing-noreturn

Change-Id: I70df6553aa715cbd99963dd66d4b4bbd55ffc64d