Return error on unsupported feature and exit

[BrandonTJones]

Summary

Within the AARCH64 Asm Parser we currently fatally exit the assembler when given a feature that exists but isn't yet supported. This change simply allows the exit to be cleaner and without a crash.

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

Operands.push_back(AArch64Operand::CreateShiftExtend(
       ShOp, MCE->getValue(), true, S, E, getContext()));
   return MatchOperand_Success;
 }
 
 static const struct Extension {
   const char *Name;
   const FeatureBitset Features;
 } ExtensionMap[] = {
+    {"dummy_extension",
+     {}}, // This is a dummy extension used for testing error outputs.
     {"crc", {AArch64::FeatureCRC}},
     {"sm4", {AArch64::FeatureSM4}},
     {"sha3", {AArch64::FeatureSHA3}},
     {"sha2", {AArch64::FeatureSHA2}},
     {"aes", {AArch64::FeatureAES}},
     {"crypto", {AArch64::FeatureCrypto}},
     {"fp", {AArch64::FeatureFPARMv8}},
     {"simd", {AArch64::FeatureNEON}},
if (Name.startswith_lower("no")) {
       Name = Name.substr(2);
     }
 
     for (const auto &Extension : ExtensionMap) {
       if (Extension.Name != Name)
         continue;
 
       if (Extension.Features.none())
-        report_fatal_error("unsupported architectural extension: " + Name);
+        return Error(ArchLoc, "unsupported architectural extension: " + Name);
 
       FeatureBitset ToggleFeatures = EnableFeature
                                          ? (~Features & Extension.Features)
                                          : ( Features & Extension.Features);
       uint64_t Features =
           ComputeAvailableFeatures(STI.ToggleFeature(ToggleFeatures));
       setAvailableFeatures(Features);
       break;

llvm/test/MC/AsmParser/AArch64/directive-feature.s

+# RUN: not llvm-mc -triple aarch64 %s 2>&1 > /dev/null| FileCheck %s --check-prefix=CHECK-ERROR
+
+# CHECK-ERROR:  error: unsupported architectural extension: dummy_extension
+TEST1:
+        .arch v8+dummy_extension