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/Basic/Targets.h b/clang/lib/Basic/Targets.h
index 6fc967d..d450aa3 100644
--- a/clang/lib/Basic/Targets.h
+++ b/clang/lib/Basic/Targets.h
@@ -1,4 +1,4 @@
-//===------- Targets.h - Declare target feature support -------------------===//
+//===------- Targets.h - Declare target feature support ---------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //