[MC] Make llvm-mc fail cleanly on invalid output asm variant.

[niravd]

Summary

Fixes PR28488.

[niravd]

Ping.

[rnk]

tools/llvm-mc/llvm-mc.cpp

567–568

I think this can fail for other reasons, such as the target not having an MCInstPrinter. I'd give a more generic error message, like "unable to create instruction printer for assembly variant N of target triple foo"

[niravd]

Changed error message.

[rnk]

lgtm

test/MC/AsmParser/invalid-asm-variant.s

+//RUN:  not llvm-mc --disassemble -triple=x86_64 --output-asm-variant=2 %s -o - 2>&1 | FileCheck %s
+
+//CHECK: error: unable to create instruction printer for target triple 'x86_64' with assembly variant 2.

tools/llvm-mc/llvm-mc.cpp

int main(int argc, char **argv) {
   std::unique_ptr<MCSubtargetInfo> STI(
       TheTarget->createMCSubtargetInfo(TripleName, MCPU, FeaturesStr));
 
   MCInstPrinter *IP = nullptr;
   if (FileType == OFT_AssemblyFile) {
     IP = TheTarget->createMCInstPrinter(Triple(TripleName), OutputAsmVariant,
                                         *MAI, *MCII, *MRI);
 
+    if (!IP) {
+      errs()
+          << "error: unable to create instruction printer for target triple '"
+          << TheTriple.normalize() << "' with assembly variant "
+          << OutputAsmVariant << ".\n";
+      return 1;
+    }
+
     // Set the display preference for hex vs. decimal immediates.
     IP->setPrintImmHex(PrintImmHex);
 
     // Set up the AsmStreamer.
     MCCodeEmitter *CE = nullptr;
     MCAsmBackend *MAB = nullptr;
     if (ShowEncoding) {
       CE = TheTarget->createMCCodeEmitter(*MCII, *MRI, Ctx);