[SystemZ/z/OS] Set DWARF version to 4 for z/OS.

The DWARF version was raised to 5 for all platforms which do not opt
out. Default to DWARF version to 4 for z/OS again.

Reviewed By: abhina.sreeskantharajan, uweigand

Differential Revision: https://reviews.llvm.org/D127498

clang/lib/Driver/ToolChains/ZOS.h

@@ -30,6 +30,8 @@ public:
 
   bool IsIntegratedAssemblerDefault() const override { return true; }
 
+  unsigned GetDefaultDwarfVersion() const override { return 4; }
+
   void addClangTargetOptions(
       const llvm::opt::ArgList &DriverArgs, llvm::opt::ArgStringList &CC1Args,
       Action::OffloadKind DeviceOffloadingKind) const override;

clang/test/Driver/zos-dwarfversion.c

@@ -0,0 +1,3 @@
+// RUN: %clang -target s390x-none-zos -g -S -emit-llvm %s -o - | FileCheck %s
+
+// CHECK: !"Dwarf Version", i32 4