[ClangImporter] Prefer serialized bridging headers in LLDB.

Especially in an explicit modules project, LLDB might not know all the
search paths needed to imported the on disk header.

rdar://157063577
(cherry picked from commit a6678476d8)

include/swift/Basic/LangOptions.h

@@ -1093,6 +1093,10 @@ namespace swift {
     /// When set, don't enforce warnings with -Werror.
     bool DebuggerSupport = false;
 
+    /// Prefer the serialized preprocessed header over the one on disk.
+    /// Used by LLDB.
+    bool PreferSerializedBridgingHeader = false;
+
     /// When set, ClangImporter is disabled, and all requests go to the
     /// DWARFImporter delegate.
     bool DisableSourceImport = false;

include/swift/ClangImporter/ClangImporter.h

@@ -163,8 +163,7 @@ private:
 
   bool requiresBuiltinHeadersInSystemModules = false;
 
-  ClangImporter(ASTContext &ctx,
-                DependencyTracker *tracker,
+  ClangImporter(ASTContext &ctx, DependencyTracker *tracker,
                 DWARFImporterDelegate *dwarfImporterDelegate);
 
   /// Creates a clone of Clang importer's compiler instance that has been
@@ -196,8 +195,8 @@ public:
   /// \returns a new Clang module importer, or null (with a diagnostic) if
   /// an error occurred.
   static std::unique_ptr<ClangImporter>
-  create(ASTContext &ctx,
-         std::string swiftPCHHash = "", DependencyTracker *tracker = nullptr,
+  create(ASTContext &ctx, std::string swiftPCHHash = "",
+         DependencyTracker *tracker = nullptr,
          DWARFImporterDelegate *dwarfImporterDelegate = nullptr,
          bool ignoreFileMapping = false);