[clang] [libcxx] Allow string to use SSO in constant evaluation. (PR #66576)

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philnik777 wrote:

@jyknight I don't think we have to worry much about that. The step count is quite arbitrary (maybe something like AST nodes?) and doesn't seem to have anything to do with the actual cost or compile time of the program. e.g. a `__builtin_memmove` of arbitrary size and a single assignment have the same step count AFAICT. You can avoid the test complexity by adding `// ADDITIONAL_COMPILE_FLAGS(has-fconstexpr-steps): -fconstexpr-steps=2000000` to the test to increase the step limit.

https://github.com/llvm/llvm-project/pull/66576