Switch from copy/remove/erase to rtrim.

[scotttodd]

Docs (https://secure.phabricator.com/book/phabricator/article/arcanist_diff/) said this would update my existing revision (https://reviews.llvm.org/D153591) :/

mlir/include/mlir/Support/IndentedOstream.h

private:
 
   /// The underlying raw_ostream.
   raw_ostream &os;
 };
 
 inline raw_indented_ostream &
 mlir::raw_indented_ostream::printReindented(StringRef str,
                                             StringRef extraPrefix) {
-  // Before splitting on \n, remove Windows \r characters from \r\n endings.
-  std::string normalizedStr = str.str();
-  normalizedStr.erase(
-      std::remove(normalizedStr.begin(), normalizedStr.end(), '\r'),
-      normalizedStr.cend());
-  StringRef output(normalizedStr);
-
+  StringRef output = str;
   // Skip empty lines.
   while (!output.empty()) {
     auto split = output.split('\n');
-    size_t indent = split.first.find_first_not_of(" \t");
+    // Trim Windows \r characters from \r\n line endings.
+    auto firstTrimmed = split.first.rtrim('\r');
+    size_t indent = firstTrimmed.find_first_not_of(" \t");
     if (indent != StringRef::npos) {
       // Set an initial value.
       leadingWs = indent;
       break;
     }
     output = split.second;
   }
   // Determine the maximum indent.

mlir/unittests/Support/IndentedOstreamTest.cpp

TEST(FormatTest, ReindentLineEndings) {
 
   // Similar string as the previous test, but with \r\n (Windows style) line
   // breaks. Note that C++'s internal string representation uses \n, so just
   // running the previous test as-is on Windows is not sufficient.
   const auto *desc =
       "\r\n\r\n\r\n         First line\r\n                 second line";
   ros.printReindented(desc);
   ros.flush();
-  const auto *expected = "First line\n        second line";
+  const auto *expected = "First line\r\n        second line";
   EXPECT_THAT(os.str(), StrEq(expected));
 }