unused-non-trivial-variable: Add option to disable the whitelist

Meaning it will warn for any unused non-trivial variable. This will
cause false-positives, such as RAII types, but still useful to run at least once
on your code.

Do use it:
export CLAZY_EXTRA_OPTIONS=unused-non-trivial-variable-no-whitelist

Next will implement blacklist/whitelist support from env-variable

CCBUG: 388979