NFC: Add the emacs c++ mode hint "-*- C++ -*-" to the headers that don't have it

https://llvm.org/docs/CodingStandards.html#file-headers

llvm-svn: 337780
diff --git a/clang/lib/Format/BreakableToken.h b/clang/lib/Format/BreakableToken.h
index d9713be..0fac8f0 100644
--- a/clang/lib/Format/BreakableToken.h
+++ b/clang/lib/Format/BreakableToken.h
@@ -1,4 +1,4 @@
-//===--- BreakableToken.h - Format C++ code -------------------------------===//
+//===--- BreakableToken.h - Format C++ code ---------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //