build: bump SwiftSemantics to "main"

[compnerd]

This allows the package to resolve with a newer toolchain. This is
required to get the Windows build enabled.

[compnerd]

This would be really nice to get merged. Its unfortunate that the CI seems to have gotten stuck somewhere. I'm hoping to figure that out, but in the mean time, this change allows building the package locally on Windows at least.

[MaxDesiatov]

Tagging a new version of SwiftSemantics and using that instead would be nicer, I guess. But overall this seems legit if temporary.

[mattt]

@compnerd The changes you need should be in SwiftSemantics 0.3.1. Can we use that tag instead of pinning to main?

[compnerd]

@mattt - this is what I get if I try .upToNextMinor(from: "0.3.1"):

error: Dependencies could not be resolved because root depends on 'SwiftSemantics' 0.3.1..<0.4.0.
'SwiftSemantics' >= 0.3.1 cannot be used because package 'swiftsemantics' is required using a stable-version but 'swiftsemantics' depends on an unstable-version package 'swift-syntax' and no versions of 'SwiftSemantics' match the requirement 0.3.2..<0.4.0.

[compnerd]

Closing in favour of #278 for signing requirements. Although that has a better commit message explaining the resolution failure so we have an alternative approach listed there.