Update slang for rebase to LLVM r235153

- Include a tiny fix to an upstream bug
  (https://llvm.org/bugs/show_bug.cgi?id=23436)
- Use BufferOutputStream instead of FormattedOutputStream in
  slang_backend

Change-Id: I9c53b6bbbcccc95513b45d8f0374ced35f7baccd
(cherry picked from commit 21cc01860b95cad7ae60c686e511e8f4ae034e39)
diff --git a/slang_backend.h b/slang_backend.h
index 1a97bd6..e3dbdef 100644
--- a/slang_backend.h
+++ b/slang_backend.h
@@ -21,7 +21,7 @@
 
 #include "llvm/IR/LegacyPassManager.h"
 
-#include "llvm/Support/FormattedStream.h"
+#include "llvm/Support/raw_ostream.h"
 
 #include "slang.h"
 #include "slang_pragma_recorder.h"
@@ -30,7 +30,7 @@
 #include "slang_version.h"
 
 namespace llvm {
-  class formatted_raw_ostream;
+  class buffer_ostream;
   class LLVMContext;
   class NamedMDNode;
   class Module;
@@ -75,7 +75,7 @@
   // Passes for code emission
   llvm::legacy::FunctionPassManager *mCodeGenPasses;
 
-  llvm::formatted_raw_ostream FormattedOutStream;
+  llvm::buffer_ostream mBufferOutStream;
 
   void CreateFunctionPasses();
   void CreateModulePasses();