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

[Paul T Robinson via llvm-commits]

https://github.com/pogo59 approved this pull request.

Hmmm normally the project prefers to minimize dependencies, pushing them down into .cpp files rather than up into .h files. But I see that MemoryBuffer.h is included by TextStubHelpers.h even though it's not used there, so in this case I accept that it's reasonable to put SmallString in the .h file.

LGTM

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