[llvm] [unittests] Add missing includes (PR #65681)

[Zero Omega via llvm-commits]

zeroomega wrote:

First time try Github pull request workflow. I hope I done it correctly.

I have a working draft GoogleTest v1.14.0 roll locally when building/testing LLVM under Linux x64. I still need to clean up the code a bit and validate on a few more platforms before I submit it for review.

The `"llvm/ADT/SmallString.h"` was included in LLVM's patch to GTest header, so it was implicitly included by the test when using LLVM's GTest fork. This causes problem when using a relatively vanilla  GTest.

The missing "using testing::DoAll" is a bit puzzling. I am not sure why the build passed without it currently. It causes build failure when using GTest v1.14.0 and I think we should add it.

https://github.com/llvm/llvm-project/pull/65681