disable DWARF5 and atomics on WebAssembly

LLVM doesn't support generating DWARF5 debugging data for WebAssembly,
and support for atomics is currently very limited.

Set LLVM target options to avoid generating DWARF5 debug data and lower
atomics to regular load/stores when building for WebAssembly.

This shouldn't affect existing platforms.

Author: zhuowei

lib/IRGen/IRGen.cpp

@@ -171,6 +171,13 @@ swift::getIRTargetOptions(const IRGenOptions &Opts, ASTContext &Ctx) {
 
   auto *Clang = static_cast<ClangImporter *>(Ctx.getClangModuleLoader());
   clang::TargetOptions &ClangOpts = Clang->getTargetInfo().getTargetOpts();
+
+  // WebAssembly doesn't support atomics or DWARF5 yet.
+  if (Clang->getTargetInfo().getTriple().isOSBinFormatWasm()) {
+    TargetOpts.DebuggerTuning = llvm::DebuggerKind::Default;
+    TargetOpts.ThreadModel = llvm::ThreadModel::Single;
+  }
+
   return std::make_tuple(TargetOpts, ClangOpts.CPU, ClangOpts.Features, ClangOpts.Triple);
 }