[Swift+WASM] [IRGen] disable debugger tuning and atomics on WebAssembly

LLVM doesn't support LLDB debugger tuning for WebAssembly
(since lldb tuning generates a DWARF5 accel table even when DWARF5 is disabled,
and WebAssembly doesn't support DWARF5).

In addition, 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

@@ -156,6 +156,14 @@ swift::getIRTargetOptions(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.
+  // (LLVM debugger tuning generates a DWARF5 accel table even when DWARF5 is disabled)
+  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);
 }