absl::flat_hash_set fails UBSan on clang 7

```
absl::flat_hash_set set = {
"s1", "s2", "s3", "s4", "s5", "s6",
"s7", "s8", "s9", "s10", "s11", "s12",
"s13", "s14", "s15"};
```
compiled with http://releases.llvm.org/7.0.0/clang+llvm-7.0.0-x86_64-apple-darwin.tar.xz
```
build:sanitize --copt=-fsanitize=address --copt=-fsanitize-address-use-after-scope
build:sanitize --cxxopt=-fsanitize=address --cxxopt=-fsanitize-address-use-after-scope
build:sanitize --linkopt=-fsanitize=address --linkopt=-fsanitize-address-use-after-scope
build:sanitize --copt=-fsanitize=undefined --copt=-fno-sanitize-recover=undefined
build:sanitize --cxxopt=-fsanitize=undefined --copt=-fno-sanitize-recover=undefined
build:sanitize --linkopt=-fsanitize=undefined --copt=-fno-sanitize-recover=undefined
```

fails with the following backtrace

```
* thread #1, queue = 'com.apple.main-thread', stop reason = Misaligned pointer use
* frame #0: 0x000000010abd0360 libclang_rt.asan_osx_dynamic.dylib`__ubsan_on_report
frame #1: 0x000000010abcb1cc libclang_rt.asan_osx_dynamic.dylib`__ubsan::Diag::~Diag() + 140
frame #2: 0x000000010abccb60 libclang_rt.asan_osx_dynamic.dylib`handleTypeMismatchImpl(__ubsan::TypeMismatchData*, unsigned long, __ubsan::ReportOptions) + 1216
frame #3: 0x000000010abccca4 libclang_rt.asan_osx_dynamic.dylib`__ubsan_handle_type_mismatch_v1_abort + 68
frame #4: 0x000000010018ec20 failure_demo`absl::little_endian::Load64(p=0x0000000107036502) at endian.h:198
frame #5: 0x000000010018de8f v`absl::hash_internal::CityHashState::Read9To16(p="parse-tree", len=10) at hash.h:757
frame #6: 0x000000010018d861 failure_demo`absl::hash_internal::CityHashState::CombineContiguousImpl(state=4417671936, first="parse-tree", len=10, (null)=integral_constant @ 0x00007ffeefbfa890) at hash.h:841
frame #7: 0x000000010018d63c failure_demo`absl::hash_internal::CityHashState::combine_contiguous(hash_state=(state_ = 4417671936), first="parse-tree", size=10) at hash.h:703
frame #8: 0x000000010018f06b failure_demo`std::__1::enable_if::value, absl::hash_internal::CityHashState>::type absl::hash_internal::hash_range_or_bytes(hash_state=(state_ = 4417671936), data="parse-tree", size=10) at hash.h:517
frame #9: 0x000000010018cdd2 failure_demo`absl::hash_internal::CityHashState absl::hash_internal::HashStateBase::combine_contiguous(state=(state_ = 4417671936), data="parse-tree", size=10) at hash.h:889
frame #10: 0x000000010018c567 failure_demo`absl::hash_internal::CityHashState absl::hash_internal::AbslHashValue(hash_state=(state_ = 4417671936), str=(__data = "parse-tree", __size = 10)) at hash.h:354
frame #11: 0x000000010018bff0 failure_demo`std::__1::enable_if<(HashSelect > >::value) == ((absl::hash_internal::InvokeHashTag)1), absl::hash_internal::CityHashState>::type absl::hash_internal::InvokeHash > >(state=(state_ = 4417671936), value=0x00007ffeefbfb460) at hash.h:655
frame #12: 0x000000010018b960 failure_demo`absl::hash_internal::CityHashState absl::hash_internal::HashStateBase::combine > >(state=(state_ = 4417671936), value=0x00007ffeefbfb460) at hash.h:881
frame #13: 0x000000010018b62f failure_demo`unsigned long absl::hash_internal::CityHashState::hash >, 0>(value=0x00007ffeefbfb460) at hash.h:723
frame #14: 0x000000010018b49f failure_demo`absl::hash_internal::HashImpl > >::operator(this=0x00007ffeefbfb480, value=0x00007ffeefbfb460)(std::__1::basic_string_view > const&) const at hash.h:871
frame #15: 0x00000001001899e9 failure_demo`absl::container_internal::StringHash::operator(this=0x0000000108c18f00, v=(__data = "parse-tree", __size = 10))(std::__1::basic_string_view >) const at hash_function_defaults.h:72
```

last executed C++ code is

```
frame #4: 0x000000010018ec20 failure_demo`absl::little_endian::Load64(p=0x0000000107036502) at endian.h:198
197 inline uint64_t Load64(const void *p) {
-> 198 return ToHost64(ABSL_INTERNAL_UNALIGNED_LOAD64(p));
199 }
```