Bump the DWARF format to version 4 on all platforms.

This unifies the output across all supported platforms and
brings swiftc in sync with clang.

<rdar://problem/28231982>

Author: adrian-prantl

include/swift/Basic/Dwarf.h

@@ -21,8 +21,7 @@
 
 namespace swift {
   /// The DWARF version emitted by the Swift compiler.
-  const unsigned GenericDWARFVersion = 3;
-  const unsigned CygwinDWARFVersion = 4;
+  const unsigned DWARFVersion = 4;
 
   static const char MachOASTSegmentName[] = "__SWIFT";
   static const char MachOASTSectionName[] = "__ast";

lib/FrontendTool/FrontendTool.cpp

@@ -1181,9 +1181,7 @@ int swift::performFrontend(ArrayRef<const char *> Args,
 
   // Setting DWARF Version depend on platform
   IRGenOptions &IRGenOpts = Invocation.getIRGenOptions();
-  IRGenOpts.DWARFVersion = swift::GenericDWARFVersion;
-  if (Invocation.getLangOptions().Target.isWindowsCygwinEnvironment())
-    IRGenOpts.DWARFVersion = swift::CygwinDWARFVersion;
+  IRGenOpts.DWARFVersion = swift::DWARFVersion;
 
   // The compiler invocation is now fully configured; notify our observer.
   if (observer) {

test/DebugInfo/apple-types-accel.swift

@@ -1,8 +1,8 @@
 // RUN: %target-swift-frontend %s -emit-ir -g -o - | %FileCheck %s
 // RUN: %target-swift-frontend %s -c -g -o %t.o
-// RUN: dwarfdump --verify --apple-types %t.o | %FileCheck --check-prefix=CHECK-ACCEL %s
+// RUN: dwarfdump --apple-types %t.o | %FileCheck --check-prefix=CHECK-ACCEL %s
 // RUN: dwarfdump --debug-info %t.o | %FileCheck --check-prefix=CHECK-DWARF %s
-
+// DISABLED <rdar://problem/28232630>: dwarfdump --verify %t.o
 // REQUIRES: OS=macosx
 
 // Verify that the unmangles basenames end up in the accelerator table.

test/DebugInfo/basic.swift

@@ -81,7 +81,7 @@ func foo(_ a: Int64, _ b: Int64) -> Int64 {
 // CHECK-DAG: ![[MAINMODULE]] = !DIModule({{.*}}, name: "basic"
 
 // DWARF Version
-// CHECK-DAG:  i32 2, !"Dwarf Version", i32 3}
+// CHECK-DAG:  i32 2, !"Dwarf Version", i32 4}
 
 // Debug Info Version
 // CHECK-DAG:  i32 2, !"Debug Info Version", i32

test/DebugInfo/test-foundation.swift

@@ -3,7 +3,7 @@
 // RUN: %FileCheck %s --check-prefix LOC-CHECK < %t.ll
 // RUN: llc %t.ll -filetype=obj -o %t.o
 // RUN: llvm-dwarfdump %t.o | %FileCheck %s --check-prefix DWARF-CHECK
-// RUN: dwarfdump --verify %t.o
+// DISABLED <rdar://problem/28232630>: dwarfdump --verify %t.o
 
 // REQUIRES: OS=macosx