Update FloatingPointTypes.swift.gyb

[own2pwn]

IMO the original message can be confusing because the Float80 type seems to be available only on MacOS.

You can reproduce this error by compiling source code that it and targets an iOS device.

[own2pwn]

@swift-ci please smoke

[stephentyrone]

@swift-ci please smoke test and merge

[own2pwn]

@stephentyrone hey Stephen, what do you think if error message would be more concrete about platform? e.g. Float80 is available only on MacOS.

[stephentyrone]

@stephentyrone hey Stephen, what do you think if error message would be more concrete about platform? e.g. Float80 is available only on MacOS.

That would be a lie, since it's also available on Linux / x86_64, right? I don't think trying to precisely define where it's available is especially useful in this error message; what matters is that it isn't available for the current platform, whatever that is.

[stephentyrone]

I.e. I like the solution in this PR.

[stephentyrone]

In case it's not obvious, the Linux build failure is unrelated to this change. I'll re-run the test once it's sorted out.

[stephentyrone]

@swift-ci please smoke test linux