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

[Zero Omega via llvm-commits]

zeroomega wrote:

> I see only two places that use DoAll in llvm/unittests, and the other one has the `using` clause, so it makes sense to have it here.
> 
> TextStubHelpers.h looks like it doesn't use SmallString, so that include should go in the files where it's actually needed.

The SmallString is used in files from `TextStubV1Tests.cpp` to `TextStubV5Tests.cpp` under the same directory of `TextStubHelpers.h`, for example: https://github.com/llvm/llvm-project/blob/main/llvm/unittests/TextAPI/TextStubV1Tests.cpp . So I think it may be better to add the missing header in the `TextStubHelpers.h`, which is shared across these unit tests.

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