Adjust incrementalCompilationCachingBasic to reproduce issue 88113

Need to not use load module interface to reproduce issue 88113 and add
a note that it needs to be fixed with prefix mapping enabled.

Author: hjyamauchi

Tests/SwiftDriverTests/Helpers/IncrementalTestHarness.swift

@@ -157,7 +157,7 @@ final class IncrementalTestHarness {
     }
   }
 
-  init(config: TestBuildConfig = .implicitModule) throws {
+  init(config: TestBuildConfig = .implicitModule, preferInterfaceModuleLoadMode: Bool = true) throws {
     self.config = config
 
     guard let sdkArgs = try Driver.sdkArgumentsForTesting() else {
@@ -204,7 +204,7 @@ final class IncrementalTestHarness {
 
     Thread.sleep(forTimeInterval: 1)
     let driver = try! Driver(args: ["swiftc"])
-    if driver.isFrontendArgSupported(.moduleLoadMode) {
+    if preferInterfaceModuleLoadMode && driver.isFrontendArgSupported(.moduleLoadMode) {
       self.extraExplicitBuildArgs = ["-Xfrontend", "-module-load-mode", "-Xfrontend", "prefer-interface"]
     }
   }

Tests/SwiftDriverTests/Helpers/TestBuildConfig.swift

@@ -65,6 +65,11 @@ enum TestBuildConfig: CaseIterable, CustomStringConvertible, Sendable {
     available([.cachingBuild, .cachingPrefixMapped])
   }
 
+  /// Not prefix mapped caching build.
+  static var cachingNonPrefixed: [TestBuildConfig] {
+    available([.cachingBuild])
+  }
+
   /// Not prefix mapped explicit build.
   static var explicitNonPrefixed: [TestBuildConfig] {
     available([.explicitModule, .cachingBuild])

Tests/SwiftDriverTests/IncrementalExplicitBuildTests.swift

@@ -42,9 +42,11 @@ struct IncrementalExplicitBuildTests: DiagVerifiable {
     try h.checkReactionToTouchingAll(checkDiagnostics: true)
   }
 
-  @Test(.serialized, arguments: TestBuildConfig.cachingConfigs)
+  // FIXME: the cachingPrefixMapped config doesn't currently work (the issue 88113)
+  @Test(.serialized, arguments: TestBuildConfig.cachingNonPrefixed)
   func incrementalCompilationCachingBasic(config: TestBuildConfig) async throws {
-    let h = try IncrementalTestHarness(config: config)
+    // Set preferInterfaceModuleLoadMode false to reproduce the issue 88113
+    let h = try IncrementalTestHarness(config: config, preferInterfaceModuleLoadMode: false)
     try h.buildInitialState(checkDiagnostics: false)
     try h.touch("other")
     try h.doABuild(