[libc] [llvm] [libc] Add user defined literals to ease testing (PR #81267)

[Nick Desaulniers via llvm-commits]

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+//===-- Unittests for user defined integer literals -----------------------===//
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nickdesaulniers wrote:

Do we need any cmake additions to be able to build+run this test via cmake?

https://github.com/llvm/llvm-project/pull/81267