[analyzer] Turn on by default the Malloc Checker and a couple of CString

[analyzer] Turn on by default the Malloc Checker and a couple of CString
checks:

- unix.Malloc - Checks for memory leaks, double free, use-after-free.
- unix.cstring.NullArg - Checks for null pointers passed as arguments to
CString functions + evaluates CString functions.
- unix.cstring.BadSizeArg - Checks for common anti-patterns in
strncat size argument.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150988 91177308-0d34-0410-b5e6-96231b3b80d8