[BPF] Replace fstream and sstream with line_iterator

[MaskRay]

Summary

This makes libLLVMBPFCodeGen.so 1128 bytes smaller for my build.

[yonghong-song]

This definitely more llvm-like. Thanks for the change!

llvm/trunk/lib/Target/BPF/BTFDebug.cpp

 #include "BTFDebug.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/CodeGen/AsmPrinter.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCObjectFileInfo.h"
 #include "llvm/MC/MCSectionELF.h"
 #include "llvm/MC/MCStreamer.h"
-#include <fstream>
-#include <sstream>
+#include "llvm/Support/LineIterator.h"
 
 using namespace llvm;
 
 static const char *BTFKindStr[] = {
 #define HANDLE_BTF_KIND(ID, NAME) "BTF_KIND_" #NAME,
 #include "BTF.def"
 };
 
std::string BTFDebug::populateFileContent(const DISubprogram *SP) {
   // No need to populate the contends if it has been populated!
   if (FileContent.find(FileName) != FileContent.end())
     return FileName;
 
   std::vector<std::string> Content;
   std::string Line;
   Content.push_back(Line); // Line 0 for empty string
 
+  std::unique_ptr<MemoryBuffer> Buf;
   auto Source = File->getSource();
-  if (Source) {
-    std::istringstream InputString(Source.getValue());
-    while (std::getline(InputString, Line))
-      Content.push_back(Line);
-  } else {
-    std::ifstream InputFile(FileName);
-    while (std::getline(InputFile, Line))
-      Content.push_back(Line);
-  }
+  if (Source)
+    Buf = MemoryBuffer::getMemBufferCopy(*Source);
+  else if (ErrorOr<std::unique_ptr<MemoryBuffer>> BufOrErr =
+               MemoryBuffer::getFile(FileName))
+    Buf = std::move(*BufOrErr);
+  if (Buf)
+    for (line_iterator I(*Buf, false), E; I != E; ++I)
+      Content.push_back(*I);
 
   FileContent[FileName] = Content;
   return FileName;
 }
 
 void BTFDebug::constructLineInfo(const DISubprogram *SP, MCSymbol *Label,
                                  uint32_t Line, uint32_t Column) {
   std::string FileName = populateFileContent(SP);