Mark PartialDiagnostic.h as a C++ header

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@94363 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/PartialDiagnostic.h

-//===--- PartialDiagnostic.h - Diagnostic "closures" ----------------------===//
+//===--- PartialDiagnostic.h - Diagnostic "closures" ------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //