Require Swift 5.8 for SPI experimental feature syntax nodes

Swift versions prior to 5.8 did not correctly
handle SPI enum cases, so avoid marking
experimental feature nodes SPI if building with
such a compiler.

Author: hamishknight

CodeGeneration/Sources/SyntaxSupport/Node.swift

@@ -98,7 +98,13 @@ public class Node {
   public func apiAttributes(forRaw: Bool = false) -> AttributeListSyntax {
     let attrList = AttributeListSyntax {
       if isExperimental {
-        "@_spi(ExperimentalLanguageFeatures)"
+        // SPI for enum cases currently requires Swift 5.8 to work correctly.
+        let experimentalSPI: AttributeListSyntax = """
+          #if compiler(>=5.8)
+          @_spi(ExperimentalLanguageFeatures)
+          #endif
+          """
+        experimentalSPI.with(\.trailingTrivia, .newline)
       }
       if forRaw {
         "@_spi(RawSyntax)"