Present != Equal, feature wise. Not by a long shot. At the same conf, one of my friends who develops the sanitizers said it's easier to develop for LLVM, so that's where Dev work is done.
It is developed by google[0] afaik. The link says it is present in LLVM and gcc. I don't know what makes the difference (may be License?). On the contrary I have felt gcc better for my work. I have been following a clang bug[1] for long and have seen no fix so far.
[0] https://github.com/google/sanitizers/wiki/AddressSanitizer