Object, COFF: Cleanup some code in getSectionName

Use StringRef::startswith to tidy up some code, no functionality change
intended.

llvm-svn: 221869
diff --git a/llvm/lib/Object/COFFObjectFile.cpp b/llvm/lib/Object/COFFObjectFile.cpp
index 1fdeae6..8d8711c 100644
--- a/llvm/lib/Object/COFFObjectFile.cpp
+++ b/llvm/lib/Object/COFFObjectFile.cpp
@@ -907,9 +907,9 @@
     Name = StringRef(Sec->Name, COFF::NameSize);
 
   // Check for string table entry. First byte is '/'.
-  if (Name[0] == '/') {
+  if (Name.startswith("/")) {
     uint32_t Offset;
-    if (Name[1] == '/') {
+    if (Name.startswith("//")) {
       if (decodeBase64StringEntry(Name.substr(2), Offset))
         return object_error::parse_failed;
     } else {