clang-format: Support #include_next

Before:
  #include_next < test.h >

After:
  #include_next <test.h>

This fixes llvm.org/PR23500

llvm-svn: 237686
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 9c3448a..d035043 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -5987,6 +5987,7 @@
                "#include \"string.h\"\n"
                "#include <a-a>\n"
                "#include < path with space >\n"
+               "#include_next <test.h>"
                "#include \"abc.h\" // this is included for ABC\n"
                "#include \"some long include\" // with a comment\n"
                "#include \"some very long include paaaaaaaaaaaaaaaaaaaaaaath\"",