[Caching] Workaround not needed modulemap not found

When caching + clang include tree is enabled, don't take module map file
from command-line in clang importer. Those are resulted from `-Xcc`
arguments and do not needed in compilation since module maps are
included in include-tree.

rdar://119577349

Author: cachemeifyoucan

include/swift/AST/DiagnosticsClangImporter.def

@@ -117,6 +117,7 @@ WARNING(nonmutating_without_mutable_fields,none,
         "attribute 'nonmutating' has no effect without any mutable fields", ())
 
 ERROR(module_map_not_found, none, "module map file '%0' not found", (StringRef))
+WARNING(module_map_ignored, none, "module map file '%0' will be ignored", (StringRef))
 
 WARNING(libc_not_found, none,
         "libc not found for '%0'; C stdlib may be unavailable",

lib/ClangImporter/ClangImporter.cpp

@@ -1133,7 +1133,12 @@ std::optional<std::vector<std::string>> ClangImporter::getClangCC1Arguments(
 
   std::vector<std::string> FilteredModuleMapFiles;
   for (auto ModuleMapFile : CI->getFrontendOpts().ModuleMapFiles) {
-    if (TempVFS->exists(ModuleMapFile)) {
+    if (ctx.ClangImporterOpts.UseClangIncludeTree) {
+      // There is no need to add any module map file here. Issue a warning and
+      // drop the option.
+      importer->Impl.diagnose(SourceLoc(), diag::module_map_ignored,
+                              ModuleMapFile);
+    } else if (TempVFS->exists(ModuleMapFile)) {
       FilteredModuleMapFiles.push_back(ModuleMapFile);
     } else {
       importer->Impl.diagnose(SourceLoc(), diag::module_map_not_found,

test/CAS/cas-explicit-module-map.swift

@@ -110,6 +110,11 @@
 
 // RUN: %target-swift-frontend -emit-module -emit-module-path %t/Foo.swiftmodule -disable-implicit-swift-modules -module-cache-path %t.module-cache -explicit-swift-module-map-file @%t/map.casid -Rmodule-loading -Xcc -Rmodule-import %s -cache-compile-job -cas-path %t/cas -allow-unstable-cache-key-for-testing 2>&1 | %FileCheck %s
 
+/// Test that if there are non-existing module-map file passed through -Xcc, this still compiles.
+// RUN: %target-swift-frontend -emit-module -emit-module-path %t/Foo.swiftmodule -disable-implicit-swift-modules -module-cache-path %t.module-cache -explicit-swift-module-map-file @%t/map.casid -Rmodule-loading -Xcc -Rmodule-import %s -cache-compile-job -cas-path %t/cas -allow-unstable-cache-key-for-testing -Xcc -fmodule-map-file=%t/do-not-exist.modulemap 2>&1 | %FileCheck %s --check-prefix=CHECK --check-prefix=MODULEMAP-IGNORE
+
+// MODULEMAP-IGNORE: warning: module map file '{{.*}}' will be ignored
+
 // RUN: %target-swift-frontend -typecheck -emit-module-interface-path %t/Foo.swiftinterface -disable-implicit-swift-modules -module-cache-path %t.module-cache -explicit-swift-module-map-file @%t/map.casid %s -cache-compile-job -cas-path %t/cas -allow-unstable-cache-key-for-testing -swift-version 5 -enable-library-evolution -o %t/Foo.swiftmodule
 // RUN: %cache-tool -cas-path %t/cas -cache-tool-action print-output-keys -- \
 // RUN:   %target-swift-frontend -typecheck -emit-module-interface-path %t/Foo.swiftinterface -disable-implicit-swift-modules \