MC: Use MCSymbol in MCObjectWriter::isWeak(), NFC

Continue to prefer `MCSymbol` when we need both.

llvm-svn: 237798
diff --git a/llvm/lib/MC/MCObjectWriter.cpp b/llvm/lib/MC/MCObjectWriter.cpp
index 6402a2f..7a9fd49 100644
--- a/llvm/lib/MC/MCObjectWriter.cpp
+++ b/llvm/lib/MC/MCObjectWriter.cpp
@@ -48,4 +48,4 @@
   return &SecA == &SecB;
 }
 
-bool MCObjectWriter::isWeak(const MCSymbolData &SD) const { return false; }
+bool MCObjectWriter::isWeak(const MCSymbol &) const { return false; }