Win64: Add the uwtable attribute by default on Win64

Now that LLVM emits correct .pdata and .xdata for inline functions, we
can reenable this.

llvm-svn: 217178
diff --git a/clang/lib/Driver/WindowsToolChain.cpp b/clang/lib/Driver/WindowsToolChain.cpp
index 913425a..ca4aa67 100644
--- a/clang/lib/Driver/WindowsToolChain.cpp
+++ b/clang/lib/Driver/WindowsToolChain.cpp
@@ -59,11 +59,10 @@
 }
 
 bool Windows::IsUnwindTablesDefault() const {
-  // FIXME: LLVM's lowering of Win64 data is broken right now.  MSVC's linker
-  // says that our object files provide invalid .pdata contributions.  Until
-  // that is fixed, don't ask for unwind tables.
-  return false;
-  //return getArch() == llvm::Triple::x86_64;
+  // 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.
+  return getArch() == llvm::Triple::x86_64;
 }
 
 bool Windows::isPICDefault() const {