[clang] [clang-format] Use fc and echo on Windows if diff is not found. (PR #205225)

[via cfe-commits]

owenca wrote:

> I'm not sure about this as on my windows system, when diff **was** found, these tests (clang-format-check-format) failed anyways. Maybe due to path differences or something like that.

Can you provide a log of the errors? It works for my x64 Native Tools Command Prompt for VS setup.

> Alternatively, the diff way is not used at all on Windows unless we can be sure it gets tested by build bots on a system where diff do exist on Windows (I know nothing about the build bot setups and configurations).

I've updated #205036 to skipping checking the format if diff is unavailable.

https://github.com/llvm/llvm-project/pull/205225