[Serialization] Always list the bridging header before any imports

This mirrors how a bridging header is processed when compiling source
files: before any of the imports. This is important for LLDB to
recreate the source environment as closely as possible to how the
compiler does it; in the test case being added involving a non-modular
header file, failure to do so resulted in a deserialization
cross-reference crash.

Note that Serialization still sorts imports, which normal resolution
of imports in source does not do. So we're still not consistent. But
this is less important than handling textual includes (bridging
headers) before modular imports.

rdar://problem/40471329

Author: jrose-apple

lib/Serialization/Serialization.cpp

@@ -1081,28 +1081,38 @@ void Serializer::writeInputBlock(const SerializationOptions &options) {
   publicImportSet.insert(publicImports.begin(), publicImports.end());
 
   removeDuplicateImports(allImports);
+
   auto clangImporter =
     static_cast<ClangImporter *>(M->getASTContext().getClangModuleLoader());
-  ModuleDecl *importedHeaderModule = clangImporter->getImportedHeaderModule();
+  ModuleDecl *bridgingHeaderModule = clangImporter->getImportedHeaderModule();
+  ModuleDecl::ImportedModule bridgingHeaderImport{{}, bridgingHeaderModule};
+
+  // Make sure the bridging header module is always at the top of the import
+  // list, mimicking how it is processed before any module imports when
+  // compiling source files.
+  if (llvm::is_contained(allImports, bridgingHeaderImport)) {
+    off_t importedHeaderSize = 0;
+    time_t importedHeaderModTime = 0;
+    std::string contents;
+    if (!options.ImportedHeader.empty()) {
+      contents = clangImporter->getBridgingHeaderContents(
+          options.ImportedHeader, importedHeaderSize, importedHeaderModTime);
+    }
+    assert(publicImportSet.count(bridgingHeaderImport));
+    ImportedHeader.emit(ScratchRecord,
+                        publicImportSet.count(bridgingHeaderImport),
+                        importedHeaderSize, importedHeaderModTime,
+                        options.ImportedHeader);
+    if (!contents.empty()) {
+      contents.push_back('\0');
+      ImportedHeaderContents.emit(ScratchRecord, contents);
+    }
+  }
+
   ModuleDecl *theBuiltinModule = M->getASTContext().TheBuiltinModule;
   for (auto import : allImports) {
-    if (import.second == theBuiltinModule)
-      continue;
-
-    if (import.second == importedHeaderModule) {
-      off_t importedHeaderSize = 0;
-      time_t importedHeaderModTime = 0;
-      std::string contents;
-      if (!options.ImportedHeader.empty())
-        contents = clangImporter->getBridgingHeaderContents(
-            options.ImportedHeader, importedHeaderSize, importedHeaderModTime);
-      ImportedHeader.emit(ScratchRecord, publicImportSet.count(import),
-                          importedHeaderSize, importedHeaderModTime,
-                          options.ImportedHeader);
-      if (!contents.empty()) {
-        contents.push_back('\0');
-        ImportedHeaderContents.emit(ScratchRecord, contents);
-      }
+    if (import.second == theBuiltinModule ||
+        import.second == bridgingHeaderModule) {
       continue;
     }
 

validation-test/Serialization/Inputs/bridging-header-first/AmbivalentProtocol.h

@@ -0,0 +1,2 @@
+@protocol AmbivalentProtocol
+@end

validation-test/Serialization/Inputs/bridging-header-first/Module/modular.h

@@ -0,0 +1 @@
+#import <AmbivalentProtocol.h>

validation-test/Serialization/Inputs/bridging-header-first/Module/module.modulemap

@@ -0,0 +1,4 @@
+module Module {
+    umbrella header "modular.h"
+    module * { export * }
+}

validation-test/Serialization/Inputs/bridging-header-first/bridging.h

@@ -0,0 +1 @@
+#import "AmbivalentProtocol.h"

validation-test/Serialization/Inputs/bridging-header-first/mangled.txt

@@ -0,0 +1 @@
+$S4main1CCACycfc

validation-test/Serialization/bridging-header-first.swift

@@ -0,0 +1,32 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-build-swift -emit-module -emit-executable %s -g -I %S/Inputs/bridging-header-first/ -import-objc-header %S/Inputs/bridging-header-first/bridging.h -o %t/main
+// RUN: llvm-bcanalyzer -dump %t/main.swiftmodule | %FileCheck -check-prefix CHECK-DUMP %s
+// RUN: %lldb-moduleimport-test %t/main -type-from-mangled %S/Inputs/bridging-header-first/mangled.txt 2>&1 | %FileCheck -check-prefix CHECK-RESOLVED-TYPE %s
+
+// RUN: %target-build-swift -emit-module -emit-executable %s -g -I %S/Inputs/bridging-header-first/ -import-objc-header %S/Inputs/bridging-header-first/bridging.h -o %t/main -whole-module-optimization
+// RUN: llvm-bcanalyzer -dump %t/main.swiftmodule | %FileCheck -check-prefix CHECK-DUMP %s
+// RUN: %lldb-moduleimport-test %t/main -type-from-mangled %S/Inputs/bridging-header-first/mangled.txt 2>&1 | %FileCheck -check-prefix CHECK-RESOLVED-TYPE %s
+
+// REQUIRES: objc_interop
+
+// CHECK-DUMP-LABEL: CONTROL_BLOCK
+// CHECK-DUMP: MODULE_NAME
+// CHECK-DUMP-SAME: 'main'
+
+// CHECK-DUMP-LABEL: INPUT_BLOCK
+// CHECK-DUMP: IMPORTED_HEADER
+// CHECK-DUMP-SAME: '{{.+}}/bridging.h'
+// CHECK-DUMP: IMPORTED_MODULE
+// CHECK-DUMP-SAME: 'Module'
+// CHECK-DUMP: IMPORTED_MODULE
+// CHECK-DUMP-SAME: 'Swift'
+
+
+// CHECK-RESOLVED-TYPE: @convention(method) (C.Type) -> () -> C
+
+import Module
+class C {}
+extension C: AmbivalentProtocol {
+  func f() {}
+}