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

[Guillaume Chatelet via llvm-commits]

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

Oh sure, I first wanted to get inputs from @michaelrj-google and @lntue. It's not quite ready for review yet.

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