[RISCV] Replace llvm_unreachable with report_fatal_error.

Parsing errors aren't handled earlier in all cases. A simple
example is llc -mtriple=riscv64 -mattr=+zve32f. If F or Finx is
not also specified, this will hit a parse error.

Use a fatal_error so that the error is conveyed to the user.

Author: topperc

llvm/lib/Target/RISCV/MCTargetDesc/RISCVTargetStreamer.cpp

@@ -48,9 +48,7 @@ void RISCVTargetStreamer::emitTargetAttributes(const MCSubtargetInfo &STI) {
   auto ParseResult = RISCVFeatures::parseFeatureBits(
       STI.hasFeature(RISCV::Feature64Bit), STI.getFeatureBits());
   if (!ParseResult) {
-    /* Assume any error about features should handled earlier.  */
-    consumeError(ParseResult.takeError());
-    llvm_unreachable("Parsing feature error when emitTargetAttributes?");
+    report_fatal_error(ParseResult.takeError());
   } else {
     auto &ISAInfo = *ParseResult;
     emitTextAttribute(RISCVAttrs::ARCH, ISAInfo->toString());