[analyzer][CStringChecker] evaluate explicit_bzero

- explicit_bzero has limited scope/usage only for security/crypto purposes but is non-optimisable version of memset/0 and bzero.
- explicit_memset has similar signature and semantics as memset but is also a non-optimisable version.

Reviewers: NoQ

Reviewed By: NoQ

Differential Revision: https://reviews.llvm.org/D54592

llvm-svn: 348884