[Clang] Remove unused -split-dwarf and obsolete -enable-split-dwarf

Summary:
The changes in D59673 made the choice redundant, since we can achieve
single-file split DWARF just by not setting an output file name.
Like llc we can also derive whether to enable Split DWARF from whether
-split-dwarf-file is set, so we don't need the flag at all anymore.

The test CodeGen/split-debug-filename.c distinguished between having set
or not set -enable-split-dwarf with -split-dwarf-file, but we can
probably just always emit the metadata into the IR.

The flag -split-dwarf wasn't used at all anymore.

Reviewers: dblaikie, echristo

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D63167

llvm-svn: 364479
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 5fe4f14..5d66473 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -473,8 +473,7 @@
   Options.EmitAddrsig = CodeGenOpts.Addrsig;
   Options.EnableDebugEntryValues = CodeGenOpts.EnableDebugEntryValues;
 
-  if (CodeGenOpts.getSplitDwarfMode() != CodeGenOptions::NoFission)
-    Options.MCOptions.SplitDwarfFile = CodeGenOpts.SplitDwarfFile;
+  Options.MCOptions.SplitDwarfFile = CodeGenOpts.SplitDwarfFile;
   Options.MCOptions.MCRelaxAll = CodeGenOpts.RelaxAll;
   Options.MCOptions.MCSaveTempLabels = CodeGenOpts.SaveTempLabels;
   Options.MCOptions.MCUseDwarfDirectory = !CodeGenOpts.NoDwarfDirectoryAsm;
@@ -864,8 +863,7 @@
     break;
 
   default:
-    if (!CodeGenOpts.SplitDwarfOutput.empty() &&
-        (CodeGenOpts.getSplitDwarfMode() == CodeGenOptions::SplitFileFission)) {
+    if (!CodeGenOpts.SplitDwarfOutput.empty()) {
       DwoOS = openOutputFile(CodeGenOpts.SplitDwarfOutput);
       if (!DwoOS)
         return;
@@ -1282,8 +1280,7 @@
     NeedCodeGen = true;
     CodeGenPasses.add(
         createTargetTransformInfoWrapperPass(getTargetIRAnalysis()));
-    if (!CodeGenOpts.SplitDwarfOutput.empty() &&
-        CodeGenOpts.getSplitDwarfMode() == CodeGenOptions::SplitFileFission) {
+    if (!CodeGenOpts.SplitDwarfOutput.empty()) {
       DwoOS = openOutputFile(CodeGenOpts.SplitDwarfOutput);
       if (!DwoOS)
         return;