Update slang for LLVM rebase to r256229

Bug: http://b/26987366

(cherry picked from commit 98cfae456bb1831336bce2b21979a04e2e31fed4)

Change-Id: Ic7f67da3fee0da075f11e3125132af7ea9c96457
diff --git a/slang_backend.h b/slang_backend.h
index ef7f635..72519ae 100644
--- a/slang_backend.h
+++ b/slang_backend.h
@@ -45,6 +45,8 @@
   class DeclGroupRef;
   class DiagnosticsEngine;
   class FunctionDecl;
+  class HeaderSearchOptions;
+  class PreprocessorOptions;
   class TagDecl;
   class TargetOptions;
   class VarDecl;
@@ -149,6 +151,8 @@
   Backend(RSContext *Context,
             clang::DiagnosticsEngine *DiagEngine,
             const RSCCOptions &Opts,
+            const clang::HeaderSearchOptions &HeaderSearchOpts,
+            const clang::PreprocessorOptions &PreprocessorOpts,
             const clang::CodeGenOptions &CodeGenOpts,
             const clang::TargetOptions &TargetOpts,
             PragmaList *Pragmas,