[llvm] [ADT] Relax iterator constraints on all_equal (PR #106400)

[via llvm-commits]

aws-taylor wrote:

@kuhar,

 I don't see any examples of using [type-parameterized tests](https://google.github.io/googletest/reference/testing.html#TYPED_TEST) in the codebase. How do y'all feel about those / would you prefer I just copy/paste the [existing tests](https://github.com/llvm/llvm-project/blob/114ff99e93edd699fd94cf2a84de75e8845fc76b/llvm/unittests/ADT/STLExtrasTest.cpp#L819)? with a new type such as `std:: forward_list` ?

https://github.com/llvm/llvm-project/pull/106400