Do not crash when enum discriminant is not recognized

Sometimes the DWARF can omit information about a discriminant, for
example when an Option shares a discriminant slot with an enum that it
wraps.  In this case, lldb could crash, because the discriminant was
not found and because there was no default variant.

No test case because this relies on a compiler bug that will soon be
fixed.

Fixes #16

lldb/source/Plugins/LanguageRuntime/Rust/RustLanguageRuntime.cpp

@@ -98,6 +98,9 @@ bool RustLanguageRuntime::GetDynamicTypeAndAddress(
     }
 
     CompilerType variant_type = ast->FindEnumVariant(type, discriminant);
+    if (!variant_type) {
+      return false;
+    }
     class_type_or_name = TypeAndOrName(variant_type);
     // The address doesn't change.
     dynamic_address.SetLoadAddress(original_ptr, exe_ctx.GetTargetPtr());

lldb/source/Symbol/RustASTContext.cpp

@@ -625,6 +625,10 @@ class RustEnum : public RustAggregateBase {
     int idx = m_default;
     if (iter != m_discriminants.end()) {
       idx = iter->second;
+    } else if (idx == -1) {
+      // If the DWARF was bad somehow, we could end up not finding the
+      // discriminant and not having a default.
+      return CompilerType();
     }
     return FieldAt(idx)->m_type;
   }