[AArch64] [Windows] Emit unwind tables by default.

[efriedma]

Summary

Unwind tables are necessary even in code that doesn't support exceptions. Among other things, the tables are used for setjmp(), and by debuggers.

Depends on D50288 so LLVM can emit the unwind tables.

[rnk]

Yep, land it after the dependency.

lib/Driver/ToolChains/MSVC.cpp

724–725

Maybe update the comment to say that Windows has required unwind tables for every new architecture since x86_32, but clang only asks LLVM to emit unwind tables for the ones that LLVM supports.

lib/Driver/ToolChains/MSVC.cpp

Tool *MSVCToolChain::buildAssembler() const {
   getDriver().Diag(clang::diag::err_no_external_assembler);
   return nullptr;
 }
 
 bool MSVCToolChain::IsIntegratedAssemblerDefault() const {
   return true;
 }
 
 bool MSVCToolChain::IsUnwindTablesDefault(const ArgList &Args) const {
-  // Emit unwind tables by default on Win64. All non-x86_32 Windows platforms
-  // such as ARM and PPC actually require unwind tables, but LLVM doesn't know
-  // how to generate them yet.
-
   // Don't emit unwind tables by default for MachO targets.
   if (getTriple().isOSBinFormatMachO())
     return false;
 
-  return getArch() == llvm::Triple::x86_64;
+  // All non-x86_32 Windows targets require unwind tables. However, LLVM
+  // doesn't know how to generate them for all targets, so only enable
+  // the ones that are actually implemented.
+  return getArch() == llvm::Triple::x86_64 ||
+         getArch() == llvm::Triple::aarch64;
 }
 
 bool MSVCToolChain::isPICDefault() const {
   return getArch() == llvm::Triple::x86_64;
 }
 
 bool MSVCToolChain::isPIEDefault() const {
   return false;