[BinaryFromat] Simplify subscript operators (NFC) (#167170)

Identified with readability-simplify-subscript-expr.

Author: kazutakahirata

llvm/unittests/BinaryFormat/MsgPackWriterTest.cpp

@@ -40,7 +40,7 @@ TEST_F(MsgPackWriter, TestWriteFixPositiveInt) {
     MPWriter.write(u);
     std::string Output = OStream.str();
     EXPECT_EQ(Output.size(), 1u);
-    EXPECT_EQ(Output.data()[0], static_cast<uint8_t>(u));
+    EXPECT_EQ(Output[0], static_cast<uint8_t>(u));
   }
 }
 
@@ -128,7 +128,7 @@ TEST_F(MsgPackWriter, TestWriteFixNegativeInt) {
     MPWriter.write(i);
     std::string Output = OStream.str();
     EXPECT_EQ(Output.size(), 1u);
-    EXPECT_EQ(static_cast<int8_t>(Output.data()[0]), static_cast<int8_t>(i));
+    EXPECT_EQ(static_cast<int8_t>(Output[0]), static_cast<int8_t>(i));
   }
 }