Emit cxx interop into DW_AT_APPLE_flags

When compiling with interop enabled, emit the C++ interop compiler flag
into the DW_AT_APPLE_flags, to make it so LLDB can accurately match the
C++ interop mode when initializing its compiler instance.

rdar://97610458
(cherry picked from commit b1dbb0a321)

include/swift/AST/IRGenOptions.h

@@ -560,13 +560,19 @@ public:
     return OptMode == OptimizationMode::ForSize;
   }
 
-  std::string getDebugFlags(StringRef PrivateDiscriminator) const {
+  std::string getDebugFlags(StringRef PrivateDiscriminator,
+                            bool EnableCXXInterop) const {
     std::string Flags = DebugFlags;
     if (!PrivateDiscriminator.empty()) {
       if (!Flags.empty())
         Flags += " ";
       Flags += ("-private-discriminator " + PrivateDiscriminator).str();
     }
+    if (EnableCXXInterop) {
+      if (!Flags.empty())
+        Flags += " ";
+      Flags += "-enable-experimental-cxx-interop";
+    }
     return Flags;
   }
 

lib/IRGen/IRGenDebugInfo.cpp

@@ -2001,9 +2001,11 @@ IRGenDebugInfoImpl::IRGenDebugInfoImpl(const IRGenOptions &Opts,
       SDK = *It;
   }
 
+  bool EnableCXXInterop =
+      IGM.getSILModule().getASTContext().LangOpts.EnableCXXInterop;
   TheCU = DBuilder.createCompileUnit(
-      Lang, MainFile, Producer, Opts.shouldOptimize(), Opts.getDebugFlags(PD),
-      MajorRuntimeVersion, SplitName,
+      Lang, MainFile, Producer, Opts.shouldOptimize(),
+      Opts.getDebugFlags(PD, EnableCXXInterop), MajorRuntimeVersion, SplitName,
       Opts.DebugInfoLevel > IRGenDebugInfoLevel::LineTables
           ? llvm::DICompileUnit::FullDebug
           : llvm::DICompileUnit::LineTablesOnly,

lib/IRGen/IRGenModule.cpp

@@ -124,13 +124,15 @@ static clang::CodeGenerator *createClangCodeGenerator(ASTContext &Context,
   case IRGenDebugInfoFormat::DWARF:
     CGO.DebugCompilationDir = Opts.DebugCompilationDir;
     CGO.DwarfVersion = Opts.DWARFVersion;
-    CGO.DwarfDebugFlags = Opts.getDebugFlags(PD);
+    CGO.DwarfDebugFlags =
+        Opts.getDebugFlags(PD, Context.LangOpts.EnableCXXInterop);
     break;
   case IRGenDebugInfoFormat::CodeView:
     CGO.EmitCodeView = true;
     CGO.DebugCompilationDir = Opts.DebugCompilationDir;
     // This actually contains the debug flags for codeview.
-    CGO.DwarfDebugFlags = Opts.getDebugFlags(PD);
+    CGO.DwarfDebugFlags =
+        Opts.getDebugFlags(PD, Context.LangOpts.EnableCXXInterop);
     break;
   }
   if (!Opts.TrapFuncName.empty()) {

test/DebugInfo/cxx_interop.swift

@@ -0,0 +1,5 @@
+// Check that the "-enable-experimental-cxx-interop" is stored as a flag in the 
+// Compile Unit.
+
+// RUN: %target-swift-frontend -emit-ir -enable-experimental-cxx-interop -g %s -o - | %FileCheck %s
+// CHECK:          !DICompileUnit({{.*}}flags: "-enable-experimental-cxx-interop"