Revert "Debug Info: Assert that the main source file name is non-empty."

This reverts commit 3fd5afc.
while investigating buildbot breakage.
rdar://problem/28457719

Author: adrian-prantl

lib/IRGen/IRGenDebugInfo.cpp

@@ -95,7 +95,7 @@ IRGenDebugInfo::IRGenDebugInfo(const IRGenOptions &Opts,
                                ClangImporter &CI,
                                IRGenModule &IGM,
                                llvm::Module &M,
-                               StringRef MainSourceFileName)
+                               SourceFile *SF)
   : Opts(Opts),
     CI(CI),
     SM(IGM.Context.SourceMgr),
@@ -107,8 +107,18 @@ IRGenDebugInfo::IRGenDebugInfo(const IRGenOptions &Opts,
     LastDebugLoc({}),
     LastScope(nullptr)
 {
-  assert(Opts.DebugInfoKind > IRGenDebugInfoKind::None &&
-         "no debug info should be generated");
+  assert(Opts.DebugInfoKind > IRGenDebugInfoKind::None
+         && "no debug info should be generated");
+  StringRef SourceFileName = SF ? SF->getFilename() :
+                                  StringRef(Opts.MainInputFilename);
+  StringRef Dir;
+  llvm::SmallString<256> AbsMainFile;
+  if (SourceFileName.empty())
+    AbsMainFile = "<unknown>";
+  else {
+    AbsMainFile = SourceFileName;
+    llvm::sys::fs::make_absolute(AbsMainFile);
+  }
 
   unsigned Lang = llvm::dwarf::DW_LANG_Swift;
   std::string Producer = version::getSwiftFullVersion(
@@ -121,14 +131,6 @@ IRGenDebugInfo::IRGenDebugInfo(const IRGenOptions &Opts,
 
   // No split DWARF on Darwin.
   StringRef SplitName = StringRef();
-
-  // The Darwin linker ld64 depends on DW_AT_comp_dir to determine
-  // whether an object file has debug info.
-  assert(!MainSourceFileName.empty() && "main source file name is empty");
-  llvm::SmallString<256> AbsMainFile;
-  AbsMainFile = MainSourceFileName;
-  llvm::sys::fs::make_absolute(AbsMainFile);
-
   // Note that File + Dir need not result in a valid path.
   // Clang is doing the same thing here.
   TheCU = DBuilder.createCompileUnit(

lib/IRGen/IRGenDebugInfo.h

@@ -97,7 +97,7 @@ class IRGenDebugInfo {
 
 public:
   IRGenDebugInfo(const IRGenOptions &Opts, ClangImporter &CI, IRGenModule &IGM,
-                 llvm::Module &M, StringRef MainSourceFileName);
+                 llvm::Module &M, SourceFile *SF);
 
   /// Finalize the llvm::DIBuilder owned by this object.
   void finalize();

lib/IRGen/IRGenModule.cpp

@@ -384,11 +384,8 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
   else
     RegisterPreservingCC = DefaultCC;
 
-  if (IRGen.Opts.DebugInfoKind > IRGenDebugInfoKind::None) {
-    StringRef MainFile(SF ? SF->getFilename()
-                          : StringRef(IRGen.Opts.MainInputFilename));
-    DebugInfo = new IRGenDebugInfo(IRGen.Opts, *CI, *this, Module, MainFile);
-  }
+  if (IRGen.Opts.DebugInfoKind > IRGenDebugInfoKind::None)
+    DebugInfo = new IRGenDebugInfo(IRGen.Opts, *CI, *this, Module, SF);
 
   initClangTypeConverter();
 }