[analyzer] Add 'optin' checker package and move localizability checkers into it.

This commit creates a new 'optin' top-level checker package and moves several of
the localizability checkers into it.

This package is for checkers that are not alpha and that would normally be on by
default but where the driver does not have enough information to determine when
they are applicable. The localizability checkers fit this criterion because the
driver cannot determine whether a project is localized or not -- this is best
determined at the IDE or build-system level.

This new package is *not* intended for checkers that are too noisy to be on by
default.

The hierarchy under 'optin' mirrors that in 'alpha': checkers under 'optin'
should be organized in the hierarchy they would have had if they were truly top
level (e.g., optin.osx.cocoa.MyOptInChecker).

Differential Revision: http://reviews.llvm.org/D14303

llvm-svn: 252080