Added LLVM_FALLTHROUGH to address warning: this statement may fall through. NFC.

llvm-svn: 304873
diff --git a/lld/lib/Core/SymbolTable.cpp b/lld/lib/Core/SymbolTable.cpp
index cacea5f..2474dcf 100644
--- a/lld/lib/Core/SymbolTable.cpp
+++ b/lld/lib/Core/SymbolTable.cpp
@@ -162,6 +162,7 @@
                    << existing->name() << " (" << existingSize << ") "
                    << newAtom.name() << " (" << newSize << ")\n";
       // fallthrough
+      LLVM_FALLTHROUGH;
     }
     case MCR_Error:
       llvm::errs() << "Duplicate symbols: "