[Object] Report an error if .alt_entry is used with ELF or COFF.

I'm looking into a better way to do this long-term, but for now at least don't
crash.

llvm-svn: 265815

llvm/lib/MC/MCELFStreamer.cpp

@@ -285,7 +285,7 @@ bool MCELFStreamer::EmitSymbolAttribute(MCSymbol *S, MCSymbolAttr Attribute) {
     break;
 
   case MCSA_AltEntry:
-    llvm_unreachable("ELF doesn't support this attribute");
+    report_fatal_error("ELF doesn't support the .alt_entry attribute");
   }
 
   return true;

llvm/lib/MC/WinCOFFStreamer.cpp

@@ -107,6 +107,8 @@ bool MCWinCOFFStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
   case MCSA_Global:
     Symbol->setExternal(true);
     break;
+  case MCSA_AltEntry:
+    report_fatal_error("COFF doesn't support the .alt_entry attribute");
   }
 
   return true;