Revert "Debug info: Make DWARF4 the default for Darwin, too."

I'm holding this change to give maintainers of Darwin buildbots more time
to update their toolchains.

This reverts commit r201375.

llvm-svn: 201520
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index ee64d42..d1e58ba 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -2660,6 +2660,9 @@
       // FIXME: we should support specifying dwarf version with
       // -gline-tables-only.
       CmdArgs.push_back("-gline-tables-only");
+      // Default is dwarf-2 for darwin.
+      if (getToolChain().getTriple().isOSDarwin())
+        CmdArgs.push_back("-gdwarf-2");
     } else if (A->getOption().matches(options::OPT_gdwarf_2))
       CmdArgs.push_back("-gdwarf-2");
     else if (A->getOption().matches(options::OPT_gdwarf_3))
@@ -2667,8 +2670,13 @@
     else if (A->getOption().matches(options::OPT_gdwarf_4))
       CmdArgs.push_back("-gdwarf-4");
     else if (!A->getOption().matches(options::OPT_g0) &&
-             !A->getOption().matches(options::OPT_ggdb0))
-      CmdArgs.push_back("-g");
+             !A->getOption().matches(options::OPT_ggdb0)) {
+      // Default is dwarf-2 for darwin.
+      if (getToolChain().getTriple().isOSDarwin())
+        CmdArgs.push_back("-gdwarf-2");
+      else
+        CmdArgs.push_back("-g");
+    }
   }
 
   // We ignore flags -gstrict-dwarf and -grecord-gcc-switches for now.