ArrayRef: Put back std::equal for operator== with a check for the empty ArrayRefs

This has the nice property of compiling down to memcmp when feasible. An empty
ArrayRef can have a nullptr in its Data field. I didn't find anything in the
standard speaking against std::equal(nullptr, nullptr, nullptr) begin valid but
MSVC asserts. The way libstdc++ lowers std::equal down to memcmp also makes
invoking std::equal with a nullptr undefined behavior so checking is the only
way to be safe.

The extra check doesn't cost us perf either because we're essentially peeling
the loop header away from the rotated loop.

llvm-svn: 230920