[clang] [-Wunsafe-buffer-usage] Add alloc_size knowledge to the 2-param span constructor warning (PR #114894)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

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

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/13202

<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 :: Support/./SupportTests.exe/6/44' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:Z:\b\llvm-clang-x86_64-sie-win\build\unittests\Support\.\SupportTests.exe-LLVM-Unit-14604-6-44.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=44 GTEST_SHARD_INDEX=6 Z:\b\llvm-clang-x86_64-sie-win\build\unittests\Support\.\SupportTests.exe
--

Script:
--
Z:\b\llvm-clang-x86_64-sie-win\build\unittests\Support\.\SupportTests.exe --gtest_filter=Caching.NoCommit
--
Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\unittests\Support\Caching.cpp(142): error: Value of: AddStream
  Actual: false
Expected: true


Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\unittests\Support\Caching.cpp:142
Value of: AddStream
  Actual: false
Expected: true



********************


```

</details>

https://github.com/llvm/llvm-project/pull/114894