Don't verify module interface from merge-module by default

Merge-module builds do not respect the -preserve-types-as-written
frontend flag and may generate broken swiftinterface files. Only verify
swiftmodule in this case if explicitly asked to.

Author: xymus

Sources/SwiftDriver/Jobs/Planning.swift

@@ -405,7 +405,14 @@ extension Driver {
        parsedOptions.hasArgument(.enableLibraryEvolution),
        parsedOptions.hasFlag(positive: .verifyEmittedModuleInterface,
                              negative: .noVerifyEmittedModuleInterface,
-                             default: true)
+                             default: true),
+
+      // Don't verify by default modules emitted from a merge-module job
+      // as it's more likely to be invalid
+      shouldCreateEmitModuleJob || compilerMode == .singleCompile ||
+        parsedOptions.hasFlag(positive: .verifyEmittedModuleInterface,
+                              negative: .noVerifyEmittedModuleInterface,
+                              default: false)
     else { return }
 
     func addVerifyJob(forPrivate: Bool) throws {

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -3958,6 +3958,16 @@ final class SwiftDriverTests: XCTestCase {
       XCTAssertEqual(plannedJobs.count, 2)
     }
 
+    // Disabled by default in merge-module
+    do {
+      var driver = try Driver(args: ["swiftc", "foo.swift", "-emit-module", "-module-name",
+                                     "foo", "-emit-module-interface",
+                                     "-enable-library-evolution",
+                                     "-no-emit-module-separately"])
+      let plannedJobs = try driver.planBuild()
+      XCTAssertEqual(plannedJobs.count, 2)
+    }
+
     // Emit-module separately
     do {
       var driver = try Driver(args: ["swiftc", "foo.swift", "-emit-module", "-module-name",