[cxx-interop] NFC: Add inline doc comments

Author: egorzhdan

include/swift/ClangImporter/ClangImporter.h

@@ -718,8 +718,13 @@ ValueDecl *getImportedMemberOperator(const DeclBaseName &name,
 } // namespace importer
 
 struct ClangInvocationFileMapping {
+  /// Mapping from a file name to an existing file path.
   SmallVector<std::pair<std::string, std::string>, 2> redirectedFiles;
+
+  /// Mapping from a file name to a string of characters that represents the
+  /// contents of the file.
   SmallVector<std::pair<std::string, std::string>, 1> overridenFiles;
+
   bool requiresBuiltinHeadersInSystemModules;
 };