[polly] Tidy uses of raw_string_ostream (NFC)

As specified in the docs,
1) raw_string_ostream is always unbuffered and
2) the underlying buffer may be used directly

( 65b1361 for further reference )

* Don't call raw_string_ostream::flush(), which is essentially a no-op.
* Avoid unneeded calls to raw_string_ostream::str(), to avoid excess indirection.

Author: JOE1994

polly/lib/Analysis/ScopDetectionDiagnostic.cpp

@@ -89,7 +89,6 @@ template <typename T> std::string operator+(Twine LHS, const T &RHS) {
   std::string Buf;
   raw_string_ostream fmt(Buf);
   fmt << RHS;
-  fmt.flush();
 
   return LHS.concat(Buf).str();
 }
@@ -669,7 +668,7 @@ std::string ReportAlias::formatInvalidAlias(std::string Prefix,
 
   OS << Suffix;
 
-  return OS.str();
+  return Message;
 }
 
 std::string ReportAlias::getRemarkName() const { return "Alias"; }

polly/lib/Analysis/ScopInfo.cpp

@@ -1818,11 +1818,9 @@ std::pair<std::string, std::string> Scop::getEntryExitStr() const {
   raw_string_ostream EntryStr(EntryName);
 
   R.getEntry()->printAsOperand(EntryStr, false);
-  EntryStr.str();
 
   if (R.getExit()) {
     R.getExit()->printAsOperand(ExitStr, false);
-    ExitStr.str();
   } else
     ExitName = "FunctionExit";
 

polly/lib/CodeGen/BlockGenerators.cpp

@@ -645,7 +645,7 @@ static std::string getInstName(Value *Val) {
   std::string Result;
   raw_string_ostream OS(Result);
   Val->printAsOperand(OS, false);
-  return OS.str();
+  return Result;
 }
 
 void BlockGenerator::generateBeginStmtTrace(ScopStmt &Stmt, LoopToScevMapT &LTS,

polly/lib/Exchange/JSONExporter.cpp

@@ -116,12 +116,12 @@ static json::Array exportArrays(const Scop &S) {
     }
     for (; i < SAI->getNumberOfDimensions(); i++) {
       SAI->getDimensionSize(i)->print(RawStringOstream);
-      Sizes.push_back(RawStringOstream.str());
+      Sizes.push_back(Buffer);
       Buffer.clear();
     }
     Array["sizes"] = std::move(Sizes);
     SAI->getElementType()->print(RawStringOstream);
-    Array["type"] = RawStringOstream.str();
+    Array["type"] = Buffer;
     Buffer.clear();
     Arrays.push_back(std::move(Array));
   }
@@ -575,14 +575,14 @@ static bool areArraysEqual(ScopArrayInfo *SAI, const json::Object &Array) {
   for (unsigned i = 1; i < Array.getArray("sizes")->size(); i++) {
     SAI->getDimensionSize(i)->print(RawStringOstream);
     const json::Array &SizesArray = *Array.getArray("sizes");
-    if (RawStringOstream.str() != SizesArray[i].getAsString().value())
+    if (Buffer != SizesArray[i].getAsString().value())
       return false;
     Buffer.clear();
   }
 
   // Check if key 'type' differs from the current one or is not valid.
   SAI->getElementType()->print(RawStringOstream);
-  if (RawStringOstream.str() != Array.getString("type").value()) {
+  if (Buffer != Array.getString("type").value()) {
     errs() << "Array has not a valid type.\n";
     return false;
   }