[XCOFF][MC] report_fatal_error before dereferencing NULL

This patch replaces a TODO comment with a call to `report_fatal_error`.
The path that reaches the added call to `report_fatal_error` manifestly
dereferences a null pointer.

llvm-svn: 368048

Author: hubert-reinterpretcast

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -728,7 +728,8 @@ AsmParser::AsmParser(SourceMgr &SM, MCContext &Ctx, MCStreamer &Out,
     PlatformParser.reset(createWasmAsmParser());
     break;
   case MCObjectFileInfo::IsXCOFF:
-    // TODO: Need to implement createXCOFFAsmParser for XCOFF format.
+    report_fatal_error(
+        "Need to implement createXCOFFAsmParser for XCOFF format.");
     break;
   }