Exclude expected completions for underscored attributes

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

Author: hamishknight

SourceKitStressTester/Sources/StressTester/ActionGenerators.swift

@@ -503,6 +503,15 @@ private struct ActionToken {
         return nil
       }
     }
+
+    // Avoid matching for underscored attributes since we hide completions for
+    // those.
+    if let attr = parent.ancestorOrSelf(mapping: { $0.as(AttributeSyntax.self) }),
+       let ident = attr.attributeName.as(IdentifierTypeSyntax.self),
+       ident.name.text.hasPrefix("_") {
+      return nil
+    }
+
     if let parent = parent.as(DeclReferenceExprSyntax.self), parent.baseName == token {
       if let refArgs = parent.argumentNames {
         let name = SwiftName(base: token.text, labels: refArgs.arguments.map{ $0.name.text })

SourceKitStressTester/Tests/StressTesterToolTests/ActionGeneratorTests.swift

@@ -97,7 +97,11 @@ class ActionGeneratorTests: XCTestCase {
 
       // FIXME: completion doesn't suggest anonymous closure params (e.g. $0)
       (#line, "$0.first", [/*"$0",*/ "first"]),
-      (#line, "[1,2,4].filter{ $0 == 4 }", ["call:filter"/*, "$0"*/])
+      (#line, "[1,2,4].filter{ $0 == 4 }", ["call:filter"/*, "$0"*/]),
+
+      // We don't include completions for underscored attributes.
+      (#line, "@_underscored(a: Foo)", []),
+      (#line, "@_spi(Foo)", []),
     ]
 
     for test in cases {