[llvm] Adjust LDBG output: surround DebugType between `[` and `]` (PR #150671)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lld-x86_64-ubuntu-fast` running on `as-builder-4` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/33/builds/20970

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM-Unit :: Support/./SupportTests/10/96' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/unittests/Support/./SupportTests-LLVM-Unit-3182833-10-96.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=96 GTEST_SHARD_INDEX=10 /home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/unittests/Support/./SupportTests
--

Script:
--
/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/build/unittests/Support/./SupportTests --gtest_filter=DebugLogTest.Basic
--
/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/unittests/Support/DebugLogTest.cpp:30: Failure
Value of: StringRef(os.str()).starts_with('[')
  Actual: false
Expected: true


/home/buildbot/worker/as-builder-4/ramdisk/lld-x86_64/llvm-project/llvm/unittests/Support/DebugLogTest.cpp:30
Value of: StringRef(os.str()).starts_with('[')
  Actual: false
Expected: true



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


```

</details>

https://github.com/llvm/llvm-project/pull/150671