Add a new path-sensitive checker for functions in <string.h>, for both null-terminate...

Add a new path-sensitive checker for functions in <string.h>, for both null-terminated strings and memory blocks. Currently only checks memcpy(), memmove(), and bcopy(), but this is intended to be expanded soon.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107722 91177308-0d34-0410-b5e6-96231b3b80d8