analyzer: Recognize __builtin_free as a matching deallocator
Recognize __builtin_free as being equivalent to free when passed into
__attribute__((malloc ())), similar to how it is treated when it is
encountered as a call. This fixes spurious warnings in glibc where
xmalloc family of allocators as well as reallocarray, memalign,
etc. are declared to have __builtin_free as the free function.