[MC] Fix likely uninitialized memory bug

[int3]

Summary

See https://reviews.llvm.org/D122258#inline-1223493. I can't repro the
issue locally but this seems like the likely culprit.

[uabelho]

As I haven't been able to reproduce the problem manually myself I can't confirm that this fixes the problem, but it does seem likely.
Thanks!

clang/tools/driver/cc1as_main.cpp

AssemblerInvocation() {
     RelaxAll = 0;
     NoExecStack = 0;
     FatalWarnings = 0;
     NoWarn = 0;
     IncrementalLinkerCompatible = 0;
     Dwarf64 = 0;
     DwarfVersion = 0;
     EmbedBitcode = 0;
+    EmitDwarfUnwind = EmitDwarfUnwindType::Default;
   }
 
   static bool CreateFromArgs(AssemblerInvocation &Res,
                              ArrayRef<const char *> Argv,
                              DiagnosticsEngine &Diags);
 };
 
 }