Avoid Abseil hash cache crashes under ASan

[VoltVoks]

Fixes #589.
Fixes #596.

This change avoids crashes in internal cache paths that use Abseil flat hash containers after extremely small memory budgets or failed cache setup leave those containers in an unsafe empty/zero-capacity state under ASan.

The two issue reproducers hit the same failure family:

• Null-deref in RE2 DFA/hash-set teardown triggered by extremely small max_mem option #589 reports a null read during DFA/hash-set teardown with tiny max_mem.
• Segfault in RE2 compiler when compiling a crafted regex #596 reports a crash while compiling a crafted UTF-8 regexp when the compiler consults its rune suffix cache.

This patch replaces the affected internal Abseil flat hash caches with standard unordered containers in the compiler and DFA cache paths, and keeps DFA::ClearCache() from iterating empty state caches. I also added regression coverage to RE2.NoCrash for both the crafted UTF-8 char-class pattern and the tiny-memory DFA teardown path.

Local verification:

ASAN_OPTIONS=detect_leaks=0 ./build-asan/re2_test --gtest_filter=RE2.NoCrash
ASAN_OPTIONS=detect_leaks=0 ./build-asan/compile_test
ASAN_OPTIONS=detect_leaks=0 timeout 30s ./build-asan/dfa_test --gtest_filter=Multithreaded.BuildEntireDFA:SingleThreaded.BuildEntireDFA:DFA.ReverseMatch:DFA.Callback
cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DRE2_TEST=ON -DRE2_BENCHMARK=OFF -DRE2_BUILD_TESTING=OFF
cmake --build build -j$(nproc)
ctest --test-dir build --output-on-failure
git diff --check

Results:

RE2.NoCrash passed under ASan.
compile_test passed under ASan.
Selected DFA tests passed under ASan.
100% tests passed, 0 tests failed out of 20.
git diff --check passed.

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