Make LLVMContext and LLVMContextImpl classes instead of structs.

llvm-svn: 78690
diff --git a/llvm/lib/Archive/ArchiveInternals.h b/llvm/lib/Archive/ArchiveInternals.h
index 98f9c89..d187ed9 100644
--- a/llvm/lib/Archive/ArchiveInternals.h
+++ b/llvm/lib/Archive/ArchiveInternals.h
@@ -31,7 +31,7 @@
 
 namespace llvm {
 
-  struct LLVMContext;
+  class LLVMContext;
 
   /// The ArchiveMemberHeader structure is used internally for bitcode
   /// archives.