[llvm] Revert changes to llvm-string stdin test and XFAIL on AIX (PR #160821)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-nvptx64-nvidia-win` running on `as-builder-8` while building `llvm` at step 7 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/155/builds/13353

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

```
Step 7 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM-Unit :: Support/./SupportTests.exe/74/103' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:C:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\build\unittests\Support\.\SupportTests.exe-LLVM-Unit-5668-74-103.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=103 GTEST_SHARD_INDEX=74 C:\buildbot\as-builder-8\llvm-nvptx64-nvidia-win\build\unittests\Support\.\SupportTests.exe
--


Note: This is test shard 75 of 103.

[==========] Running 16 tests from 16 test suites.

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

[----------] 1 test from Base64Test

[ RUN      ] Base64Test.Base64

[       OK ] Base64Test.Base64 (0 ms)

[----------] 1 test from Base64Test (0 ms total)



[----------] 1 test from CommandLineTest

[ RUN      ] CommandLineTest.TokenizeGNUCommandLine

[       OK ] CommandLineTest.TokenizeGNUCommandLine (0 ms)

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



[----------] 1 test from FPUNumberModule

[ RUN      ] FPUNumberModule.testAttribute

[       OK ] FPUNumberModule.testAttribute (0 ms)

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



[----------] 1 test from Error

[ RUN      ] Error.HasValueMatcher

[       OK ] Error.HasValueMatcher (0 ms)

[----------] 1 test from Error (0 ms total)
...

```

</details>

https://github.com/llvm/llvm-project/pull/160821