[analyzer] Move zero-size allocation checks to optin.portability.

This is a new checker package. It contains checkers that highlight
well-documented implementation-defined behavior. Such checkers are only useful
to developers that intend to write portable code. Code that is only compiled for
a single platform should be allowed to rely on this platform's specific
documented behavior.

rdar://problem/30545046

Differential Revision: https://reviews.llvm.org/D34102

llvm-svn: 306396