[llvm] [NFC][ADT] Add RadixTree (PR #164524)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-expensive-checks-win` running on `as-worker-93` while building `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/14/builds/4537

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM-Unit :: ADT/./ADTTests.exe/1/63' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:C:\a\llvm-clang-x86_64-expensive-checks-win\build\unittests\ADT\.\ADTTests.exe-LLVM-Unit-15372-1-63.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=63 GTEST_SHARD_INDEX=1 C:\a\llvm-clang-x86_64-expensive-checks-win\build\unittests\ADT\.\ADTTests.exe
--

Note: This is test shard 2 of 63.

[==========] Running 32 tests from 30 test suites.

[----------] Global test environment set-up.

[----------] 1 test from AnyTest

[ RUN      ] AnyTest.ConstructionAndAssignment

[       OK ] AnyTest.ConstructionAndAssignment (0 ms)

[----------] 1 test from AnyTest (0 ms total)



[----------] 2 tests from APFloatTest

[ RUN      ] APFloatTest.isNegative

[       OK ] APFloatTest.isNegative (4 ms)

[ RUN      ] APFloatTest.Float8E4M3FNUZFromString

[       OK ] APFloatTest.Float8E4M3FNUZFromString (1 ms)

[----------] 2 tests from APFloatTest (6 ms total)



[----------] 2 tests from APIntTest

[ RUN      ] APIntTest.PowZeroTo5

[       OK ] APIntTest.PowZeroTo5 (0 ms)

[ RUN      ] APIntTest.insertBits

[       OK ] APIntTest.insertBits (0 ms)

[----------] 2 tests from APIntTest (1 ms total)



...

```

</details>

https://github.com/llvm/llvm-project/pull/164524