[llvm] Replace SmallStr.str().str() with std::string conversion operator.

Use the std::string conversion operator introduced in
d7049213d0fcda691c9e79f9b41e357198d99738.
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
index 6e87c70..8191ec8 100644
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -1357,53 +1357,53 @@
   bool isSigned;
 
   APInt(8, 0).toString(S, 2, true, true);
-  EXPECT_EQ(S.str().str(), "0b0");
+  EXPECT_EQ(std::string(S), "0b0");
   S.clear();
   APInt(8, 0).toString(S, 8, true, true);
-  EXPECT_EQ(S.str().str(), "00");
+  EXPECT_EQ(std::string(S), "00");
   S.clear();
   APInt(8, 0).toString(S, 10, true, true);
-  EXPECT_EQ(S.str().str(), "0");
+  EXPECT_EQ(std::string(S), "0");
   S.clear();
   APInt(8, 0).toString(S, 16, true, true);
-  EXPECT_EQ(S.str().str(), "0x0");
+  EXPECT_EQ(std::string(S), "0x0");
   S.clear();
   APInt(8, 0).toString(S, 36, true, false);
-  EXPECT_EQ(S.str().str(), "0");
+  EXPECT_EQ(std::string(S), "0");
   S.clear();
 
   isSigned = false;
   APInt(8, 255, isSigned).toString(S, 2, isSigned, true);
-  EXPECT_EQ(S.str().str(), "0b11111111");
+  EXPECT_EQ(std::string(S), "0b11111111");
   S.clear();
   APInt(8, 255, isSigned).toString(S, 8, isSigned, true);
-  EXPECT_EQ(S.str().str(), "0377");
+  EXPECT_EQ(std::string(S), "0377");
   S.clear();
   APInt(8, 255, isSigned).toString(S, 10, isSigned, true);
-  EXPECT_EQ(S.str().str(), "255");
+  EXPECT_EQ(std::string(S), "255");
   S.clear();
   APInt(8, 255, isSigned).toString(S, 16, isSigned, true);
-  EXPECT_EQ(S.str().str(), "0xFF");
+  EXPECT_EQ(std::string(S), "0xFF");
   S.clear();
   APInt(8, 255, isSigned).toString(S, 36, isSigned, false);
-  EXPECT_EQ(S.str().str(), "73");
+  EXPECT_EQ(std::string(S), "73");
   S.clear();
 
   isSigned = true;
   APInt(8, 255, isSigned).toString(S, 2, isSigned, true);
-  EXPECT_EQ(S.str().str(), "-0b1");
+  EXPECT_EQ(std::string(S), "-0b1");
   S.clear();
   APInt(8, 255, isSigned).toString(S, 8, isSigned, true);
-  EXPECT_EQ(S.str().str(), "-01");
+  EXPECT_EQ(std::string(S), "-01");
   S.clear();
   APInt(8, 255, isSigned).toString(S, 10, isSigned, true);
-  EXPECT_EQ(S.str().str(), "-1");
+  EXPECT_EQ(std::string(S), "-1");
   S.clear();
   APInt(8, 255, isSigned).toString(S, 16, isSigned, true);
-  EXPECT_EQ(S.str().str(), "-0x1");
+  EXPECT_EQ(std::string(S), "-0x1");
   S.clear();
   APInt(8, 255, isSigned).toString(S, 36, isSigned, false);
-  EXPECT_EQ(S.str().str(), "-1");
+  EXPECT_EQ(std::string(S), "-1");
   S.clear();
 }