[CStringSyntaxChecker] Check strlcat sizeof check

Assuming strlcat is used with strlcpy we check as we can if the last argument does not equal os not larger than the buffer.
Advising the proper usual pattern.

Reviewers: george.karpenkov, NoQ, MaskRay

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D49722

llvm-svn: 342832