Fix MacOS crash which involves two static absl::flat_hash_set globals.

Details:

```shell
Assertion failed: (ctrl != nullptr), function iterator, file external/com_google_absl/absl/container/internal/raw_hash_set.h, line 749.
Process 70835 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = hit program assert
    frame #4: 0x000000010000ae22 utf8_delimiters_test`absl::container_internal::raw_hash_set<absl::container_internal::FlatHashSetPolicy<char32_t>, absl::hash_internal::Hash<char32_t>, std::__1::equal_to<char32_t>, std::__1::allocator<char32_t> >::iterator::iterator(signed char*, char32_t*) + 98
utf8_delimiters_test`absl::container_internal::raw_hash_set<absl::container_internal::FlatHashSetPolicy<char32_t>, absl::hash_internal::Hash<char32_t>, std::__1::equal_to<char32_t>, std::__1::allocator<char32_t> >::iterator::iterator:
->  0x10000ae22 <+98>:  jmp    0x10000ae27               ; <+103>
    0x10000ae27 <+103>: addq   $0x20, %rsp
    0x10000ae2b <+107>: popq   %rbp
    0x10000ae2c <+108>: retq
Target 0: (utf8_delimiters_test) stopped.
(lldb) up
```

PiperOrigin-RevId: 382336985

nisaba/port/unicode_properties.cc

@@ -21,7 +21,7 @@ namespace nisaba {
 namespace utf8 {
 namespace {
 
-const absl::flat_hash_set<char32_t> kBreakingWhitespace = {
+constexpr char32_t kBreakingWhitespace[] = {
   U'\u0009',  // character tabulation
   U'\u000A',  // line feed
   U'\u000B',  // line tabulation
@@ -49,7 +49,7 @@ const absl::flat_hash_set<char32_t> kBreakingWhitespace = {
   U'\u3000',  // ideographic space
 };
 
-const absl::flat_hash_set<char32_t> kNonBreakingWhitespace = {
+constexpr char32_t kNonBreakingWhitespace[] = {
   U'\u180E',  // mongolian vowel separator
   U'\u200B',  // zero width space
   U'\u200C',  // zero width non-joiner
@@ -61,17 +61,20 @@ const absl::flat_hash_set<char32_t> kNonBreakingWhitespace = {
 }  // namespace
 
 absl::flat_hash_set<char32_t> GetBreakingWhitespaceChars() {
-  return kBreakingWhitespace;
+  return absl::flat_hash_set<char32_t>(std::begin(kBreakingWhitespace),
+                                       std::end(kBreakingWhitespace));
 }
 
 absl::flat_hash_set<char32_t> GetNonBreakingWhitespaceChars() {
-  return kNonBreakingWhitespace;
+  return absl::flat_hash_set<char32_t>(std::begin(kNonBreakingWhitespace),
+                                       std::end(kNonBreakingWhitespace));
 }
 
 absl::flat_hash_set<char32_t> GetAllWhitespaceChars() {
   absl::flat_hash_set<char32_t> all_chars;
-  std::set_union(kBreakingWhitespace.begin(), kBreakingWhitespace.end(),
-                 kNonBreakingWhitespace.begin(), kNonBreakingWhitespace.end(),
+  std::set_union(std::begin(kBreakingWhitespace), std::end(kBreakingWhitespace),
+                 std::begin(kNonBreakingWhitespace),
+                 std::end(kNonBreakingWhitespace),
                  std::inserter(all_chars, all_chars.begin()));
   return all_chars;
 }