Don't use report_fatal_error in the LoadBorrowImmutabilityChecker.

That prevents the SILVerifier from printing the context, making it
hard to quickly produce test cases from logs.

Instead, just return the failure status to the SILVerifier so it can
do its diagnostic thing.

An assert or llvm_unreachable would also be fine in addition to the
normal SILVerifier diagnostics, but I don't think that's needed here.

Author: atrick

lib/SIL/Verifier/LoadBorrowImmutabilityChecker.cpp

@@ -267,12 +267,11 @@ bool GatherWritesVisitor::visitUse(Operand *op, AccessUseType useTy) {
   if (!op->get()->getType().isAddress() && !user->mayWriteToMemory()) {
     return true;
   }
-  // If we did not recognize the user, just return conservatively that it was
-  // written to in a way we did not understand.
+  // If we did not recognize the user, print additional error diagnostics and
+  // return false to force SIL verification to fail.
   llvm::errs() << "Function: " << user->getFunction()->getName() << "\n";
   llvm::errs() << "Value: " << op->get();
   llvm::errs() << "Unknown instruction: " << *user;
-  llvm::report_fatal_error("Unexpected instruction using borrowed address?!");
   return false;
 }