[llvm] Avoid 'raw_string_ostream::str()' (NFC) (#97203)

Since `raw_string_ostream` doesn't own the string buffer, it is
desirable (in terms of memory safety) for users to directly reference
the string buffer rather than use `raw_string_ostream::str()`.

Work towards TODO comment to remove `raw_string_ostream::str()`.

Author: JOE1994

llvm/include/llvm/DebugInfo/DWARF/DWARFContext.h

@@ -422,7 +422,7 @@ class DWARFContext : public DIContext {
     for (unsigned Size : DWARFContext::getSupportedAddressSizes())
       Stream << LS << Size;
     Stream << ')';
-    return make_error<StringError>(Stream.str(), EC);
+    return make_error<StringError>(Buffer, EC);
   }
 
   std::shared_ptr<DWARFContext> getDWOContext(StringRef AbsolutePath);

llvm/include/llvm/Support/Error.h

@@ -54,7 +54,7 @@ class ErrorInfoBase {
     std::string Msg;
     raw_string_ostream OS(Msg);
     log(OS);
-    return OS.str();
+    return Msg;
   }
 
   /// Convert this error to a std::error_code.
@@ -761,7 +761,7 @@ inline void cantFail(Error Err, const char *Msg = nullptr) {
     std::string Str;
     raw_string_ostream OS(Str);
     OS << Msg << "\n" << Err;
-    Msg = OS.str().c_str();
+    Msg = Str.c_str();
 #endif
     llvm_unreachable(Msg);
   }
@@ -792,7 +792,7 @@ T cantFail(Expected<T> ValOrErr, const char *Msg = nullptr) {
     raw_string_ostream OS(Str);
     auto E = ValOrErr.takeError();
     OS << Msg << "\n" << E;
-    Msg = OS.str().c_str();
+    Msg = Str.c_str();
 #endif
     llvm_unreachable(Msg);
   }
@@ -823,7 +823,7 @@ T& cantFail(Expected<T&> ValOrErr, const char *Msg = nullptr) {
     raw_string_ostream OS(Str);
     auto E = ValOrErr.takeError();
     OS << Msg << "\n" << E;
-    Msg = OS.str().c_str();
+    Msg = Str.c_str();
 #endif
     llvm_unreachable(Msg);
   }
@@ -1338,7 +1338,7 @@ class FileError final : public ErrorInfo<FileError> {
     std::string Msg;
     raw_string_ostream OS(Msg);
     Err->log(OS);
-    return OS.str();
+    return Msg;
   }
 
   StringRef getFileName() const { return FileName; }

llvm/include/llvm/Support/GraphWriter.h

@@ -228,9 +228,9 @@ class GraphWriter {
           O << "|";
 
       if (RenderUsingHTML)
-        O << EdgeSourceLabels.str();
+        O << edgeSourceLabels;
       else
-        O << "{" << EdgeSourceLabels.str() << "}";
+        O << "{" << edgeSourceLabels << "}";
 
       if (DTraits.renderGraphFromBottomUp())
         if (!RenderUsingHTML)

llvm/include/llvm/Support/ScopedPrinter.h

@@ -86,7 +86,7 @@ template <class T> std::string to_string(const T &Value) {
   std::string number;
   raw_string_ostream stream(number);
   stream << Value;
-  return stream.str();
+  return number;
 }
 
 template <typename T, typename TEnum>

llvm/include/llvm/Support/YAMLTraits.h

@@ -1026,7 +1026,7 @@ yamlize(IO &YamlIO, T &Val, bool, EmptyContext &Ctx) {
     std::string Storage;
     raw_string_ostream Buffer(Storage);
     BlockScalarTraits<T>::output(Val, YamlIO.getContext(), Buffer);
-    StringRef Str = Buffer.str();
+    StringRef Str(Storage);
     YamlIO.blockScalarString(Str);
   } else {
     StringRef Str;
@@ -1046,8 +1046,8 @@ yamlize(IO &io, T &Val, bool, EmptyContext &Ctx) {
     raw_string_ostream ScalarBuffer(ScalarStorage), TagBuffer(TagStorage);
     TaggedScalarTraits<T>::output(Val, io.getContext(), ScalarBuffer,
                                   TagBuffer);
-    io.scalarTag(TagBuffer.str());
-    StringRef ScalarStr = ScalarBuffer.str();
+    io.scalarTag(TagStorage);
+    StringRef ScalarStr(ScalarStorage);
     io.scalarString(ScalarStr,
                     TaggedScalarTraits<T>::mustQuote(Val, ScalarStr));
   } else {