[clang] [llvm] Triple: Forward declare Twine and remove include (PR #145685)

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

This commit has been identified as causing a change in Clang driver behaviour. The observed symptom is that the Clang driver is passing `unknown` to `-cc1` instead of the configured default target triple of `powerpc-ibm-aix7.3.0.0`. Investigation is ongoing but we may need to revert this due to major impacts to our internal environment.

https://github.com/llvm/llvm-project/pull/145685