Replace std::ofstream with llvm::raw_fd_ostream in -verify-apply-fixes (#15133)

This isn't a user-facing feature, but why bother using std::ofstream?

Author: jrose-apple

lib/Frontend/DiagnosticVerifier.cpp

@@ -17,9 +17,10 @@
 #include "swift/Frontend/DiagnosticVerifier.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/Parse/Lexer.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/raw_ostream.h"
-#include <fstream>
+
 using namespace swift;
 
 namespace {
@@ -688,9 +689,12 @@ void DiagnosticVerifier::autoApplyFixes(unsigned BufferID,
 
   // Retain the end of the file.
   Result.append(LastPos, bufferRange.end());
-  
-  std::ofstream outs(memBuffer->getBufferIdentifier());
-  outs << Result;
+
+  std::error_code error;
+  llvm::raw_fd_ostream outs(memBuffer->getBufferIdentifier(), error,
+                            llvm::sys::fs::OpenFlags::F_None);
+  if (!error)
+    outs << Result;
 }
 
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