[analyzer] Improve the modeling of memset().

Since there is no perfect way bind the non-zero value with the default binding, this patch only considers the case where buffer's offset is zero and the char value is 0. And according to the value for overwriting, decide how to update the string length.

Reviewers: dcoughlin, NoQ, xazax.hun, a.sidorin, george.karpenkov

Reviewed By: NoQ

Differential Revision: https://reviews.llvm.org/D44934

llvm-svn: 332463