[analyzer] Add taint transfer by strcpy & others (part 1).

To simplify the process:
Refactor taint generation checker to simplify passing the
information on which arguments need to be tainted from pre to post
visit.

Todo: We need to factor out the code that sema is using to identify the
string and memcpy functions and use it here and in the CString checker.

llvm-svn: 148010