[rustllvm] Use report_fatal_error over llvm_unreachable

This makes it more robust when assertions are disabled,
crashing instead of causing UB.

Also introduces a tidy check to enforce this rule,
which in turn necessitated making tidy run on src/rustllvm.

Fixes #44020

src/rustllvm/ArchiveWrapper.cpp

@@ -66,7 +66,7 @@ static Archive::Kind fromRust(LLVMRustArchiveKind Kind) {
   case LLVMRustArchiveKind::COFF:
     return Archive::K_COFF;
   default:
-    llvm_unreachable("Bad ArchiveKind.");
+    report_fatal_error("Bad ArchiveKind.");
   }
 }