Attempt to migrate default dwarf version to 4 for linux.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189823 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 819ed4b..7e21f55 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -334,8 +334,8 @@
   else if (Args.hasArg(OPT_gdwarf_4))
     Opts.DwarfVersion = 4;
   else if (Opts.getDebugInfo() != CodeGenOptions::NoDebugInfo)
-    // Default Dwarf version is 3 if we are generating debug information.
-    Opts.DwarfVersion = 3;
+    // Default Dwarf version is 4 if we are generating debug information.
+    Opts.DwarfVersion = 4;
 
   Opts.DisableLLVMOpts = Args.hasArg(OPT_disable_llvm_optzns);
   Opts.DisableRedZone = Args.hasArg(OPT_disable_red_zone);