Avoid dynamic mapping to self for reverse macro dependency

Commit 04f1c92537 added a mechanism that
dynamically adds a mapping from the includee to the includer in this
case:

    // a.h
    #if CONTROL_FLAG
    void foo();
    #else
    void bar();
    #endif

    // b.h
    #define CONTROL_FLAG
    #include "a.h"

However, there was no check that includee and includer were different,
so regular #include guards would cause self-mappings, which would
eventually lead to cycles in mappings and assertion failures, e.g.:

    Cycle in include-mapping:
      <z.hpp> ->
      <z.hpp>
    ,,,/iwyu_include_picker.cc:845: Assertion failed: Cycle in include-mapping
    Aborted

Avoid this by checking if the file defining the macro is the same as the
file using it; in that case we don't need a mapping.

iwyu_output.cc

@@ -2120,6 +2120,10 @@ void IwyuFileInfo::HandlePreprocessingDone() {
                       << " as public header for " << private_include
                       << " because latter is already marked as public,"
                       << " though uses macro defined by includer.\n";
+      } else if (file_ == macro_use_includee) {
+        ERRSYM(file_) << "Skip marking " << quoted_file_
+                      << " as public header for " << private_include
+                      << " because they are the same file.\n";
       } else {
         ERRSYM(file_) << "Mark " << quoted_file_
                       << " as public header for " << private_include

tests/cxx/self_map_cycle-d1.h

@@ -0,0 +1,18 @@
+//===--- self_map_cycle-d1.h - test input file for iwyu -------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef INCLUDE_WHAT_YOU_USE_TESTS_CXX_SELF_MAP_CYCLE_D1_H_
+#define INCLUDE_WHAT_YOU_USE_TESTS_CXX_SELF_MAP_CYCLE_D1_H_
+
+#include "tests/cxx/self_map_cycle-d1.h"
+
+struct X {
+};
+
+#endif

tests/cxx/self_map_cycle.cc

@@ -0,0 +1,24 @@
+//===--- self_map_cycle.cc - test input file for iwyu ---------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// IWYU_ARGS: -I .
+
+// Test that self-includes do not create a mapping cycle, which would later be
+// detected and abort with assertion failure (as described in issue #424).
+
+// Note that "" #include was required to trigger the assertion here.
+#include "tests/cxx/self_map_cycle-d1.h"
+
+X x;
+
+/**** IWYU_SUMMARY
+
+(tests/cxx/self_map_cycle.cc has correct #includes/fwd-decls)
+
+***** IWYU_SUMMARY */