Add memcmp() and bcmp() to CStringChecker. These check for valid access to the buffer...

Add memcmp() and bcmp() to CStringChecker. These check for valid access to the buffer arguments and have a special-case for when the buffer arguments are known to be the same address, or when the size is zero.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107761 91177308-0d34-0410-b5e6-96231b3b80d8