[5.9] Fix Triple formatting in error messages (#6545)

* Fix `Triple` formatting in error messages

Currently `Triple` is not `CustomStringConvertible`, which leads to issues when using it in string interpolations. It uses default implementation that prints all of the triple components separately, which isn't user-readable.

rdar://107882041

* Fix `TripleTests.swift` build issue

Author: MaxDesiatov

Sources/Basics/Triple.swift

@@ -298,6 +298,10 @@ extension Triple {
     }
 }
 
+extension Triple: CustomStringConvertible {
+    public var description: String { tripleString }
+}
+
 extension Triple.Error: CustomNSError {
     public var errorUserInfo: [String: Any] {
         [NSLocalizedDescriptionKey: "\(self)"]

Tests/BasicsTests/TripleTests.swift

@@ -0,0 +1,21 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Basics
+import XCTest
+
+final class TripleTests: XCTestCase {
+    func testDescription() throws {
+        let triple = try Triple("x86_64-pc-linux-gnu")
+        XCTAssertEqual("foo \(triple) bar", "foo x86_64-pc-linux-gnu bar")
+    }
+}