[analyzer] MallocSizeofChecker false positive: when sizeof is argument

[analyzer] MallocSizeofChecker false positive: when sizeof is argument
to addition.

We should not to warn in case the malloc size argument is an
addition containing 'sizeof' operator - it is common to use the pattern
to pack values of different sizes into a buffer.

Ex:

uint8_t *buffer = (uint8_t*)malloc(dataSize + sizeof(length));

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158219 91177308-0d34-0410-b5e6-96231b3b80d8