remove unneeded forward decl.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@66151 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/clang/Basic/Diagnostic.h b/include/clang/Basic/Diagnostic.h
index 6e8ef26..1bb294e 100644
--- a/include/clang/Basic/Diagnostic.h
+++ b/include/clang/Basic/Diagnostic.h
@@ -25,7 +25,6 @@
 namespace clang {
   class DiagnosticClient;
   class SourceRange;
-  class SourceManager;
   class DiagnosticBuilder;
   class IdentifierInfo;