Revert "clang-cl: Hook up /Za to prevent adding oldnames.lib dependency"

This reverts commit r187991 and adjusts the comment.  /Za is much more
involved, and we don't want to give anyone the impression we actually
support it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187998 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index a1193f5..83e6aed 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -3719,11 +3719,10 @@
       llvm_unreachable("Unexpected option ID.");
   }
 
-  if (!Args.hasArg(options::OPT__SLASH_Za)) {
-    // This provides POSIX compatibility (maps 'open' to '_open'),
-    // which most users want.
-    CmdArgs.push_back("--dependent-lib=oldnames");
-  }
+  // This provides POSIX compatibility (maps 'open' to '_open'), which most
+  // users want.  The /Za flag to cl.exe turns this off, but it's not
+  // implemented in clang.
+  CmdArgs.push_back("--dependent-lib=oldnames");
 }
 
 void ClangAs::ConstructJob(Compilation &C, const JobAction &JA,