Exclude expected completions for underscored attributes

Don't check completions for e.g `@_spi` or its arguments.

Author: hamishknight

SourceKitStressTester/Sources/StressTester/ActionGenerators.swift

@@ -529,6 +529,31 @@ private struct ActionToken {
       }
     }
 
+    // Avoid matching for underscored attributes since we hide completions for
+    // those.
+    do {
+      var parent = parent
+      if let label = parent.as(LabeledExprSyntax.self),
+         let labelParent = label.parent, label.label == token {
+        parent = labelParent
+      } else if let nextParent = parent.parent {
+        parent = nextParent
+        if let label = parent.as(LabeledExprSyntax.self),
+           let labelParent = label.parent {
+          parent = labelParent
+        }
+      }
+      if let exprList = parent.as(LabeledExprListSyntax.self),
+          let listParent = exprList.parent {
+        parent = listParent
+      }
+      if let attr = parent.as(AttributeSyntax.self),
+         let ident = attr.attributeName.as(IdentifierTypeSyntax.self),
+         ident.name.text.hasPrefix("_") {
+        return nil
+      }
+    }
+
     let result = expectedResultImpl(for: token, parent: parent)
 
     // If the argument labels are passing along `file` and `line` arguments,