[swift-dev] Support only ninja built from source

[Wojciech Czekalski]

This is a humble proposal (or rather thought) to drop support for ninja which does not come from source.

This is motivated by two things:
1. I just wasted an hour figuring out what’s wrong, and applying workarounds
2. I *think* (please correct me if I’m wrong), there are no disadvantages of removing it, it will just simplify implementation of scripts.

I ran into it running `build-toolchain` script and it couldn’t find ninja. It came out that `build-toolchain` uses a preset which makes `build-script` actually build ninja.

Since it is not realistically possible to document everything in a big project like this, I think that gradually removing complexity (even if it is marginal) is a good thing.

Are there any disadvantages? What do you think?