SwiftPM outputs raw diagnostics from SwiftDriver without trailing newlines

SwiftPM uses the Swift Driver's `-parseable-output` flag to get a sequence of length-prefixed JSON-encoded messages that it can read.  Unfortunately the Swift Driver doesn't JSON-encode its own diagnostics, leading to things like #5968.

I filed #5968 on the Swift Driver to get it to encode its JSON messages, but meanwhile, it turns out that we can fairly easily fix the fallback logic that SwiftPM uses when it emits raw output.  It was simply missing a newline.  It's safe to always add one, since the logic that parses JSON splits by newlines, so we won't find any terminating newlines.

rdar://103608636

Author: abertelrud

Sources/Build/BuildOperationBuildSystemDelegateHandler.swift

@@ -773,7 +773,7 @@ final class BuildOperationBuildSystemDelegateHandler: LLBuildBuildSystemDelegate
                     self.progressAnimation.clear()
                 }
 
-                self.outputStream <<< output
+                self.outputStream <<< output <<< "\n"
                 self.outputStream.flush()
 
                 // next we want to try and scoop out any errors from the output (if reasonable size, otherwise this will be very slow),

Tests/CommandsTests/BuildToolTests.swift

@@ -390,4 +390,13 @@ final class BuildToolTests: CommandsTestCase {
             XCTAssertMatch(output, .prefix("digraph Jobs {"))
         }
     }
+    
+    func testSwiftDriverRawOutputGetsNewlines() throws {
+        try fixture(name: "DependencyResolution/Internal/Simple") { fixturePath in
+            // Building with `-wmo` should result in a `remark: Incremental compilation has been disabled: it is not compatible with whole module optimization` message, which should have a trailing newline.
+            let result = try execute(["-c", "release", "-Xswiftc", "-wmo"], packagePath: fixturePath)
+            XCTAssertMatch(result.stdout, .contains("optimization\n["))
+            XCTAssertNoMatch(result.stdout, .contains("optimization["))
+        }
+    }
 }