Update checkout for swift-syntax.
Update checkout for swift-syntax to HEAD commit to fix build error on macOS:
https://github.com/apple/swift-syntax/commit/3e3eb191fcdbecc6031522660c4ed6ce25282c25
```
swift-syntax/Sources/SwiftSyntax/SourceLocation.swift:410:15:
error: pattern cannot match values of type 'TriviaPiece'
let .backticks(count):
~^~~~~~~~~~~~~~~~
swift-syntax/Sources/SwiftSyntax/SourceLocation.swift:410:25:
error: pattern variable bound to type '<<error type>>', expected type 'Int'
let .backticks(count):
^
```
