[SIL] Don't abort via llvm_unreachable.

It is a way to indicate to the compiler that the line can't be reached.
On some configurations, it does happen to trap, as desired.  Use a
function that always traps, since that's the intent.

Author: nate-chandler

lib/SIL/Verifier/LinearLifetimeCheckerPrivate.h

@@ -208,7 +208,7 @@ class LLVM_LIBRARY_VISIBILITY LinearLifetimeChecker::ErrorBuilder {
     }
 
     llvm::errs() << "Found ownership error?!\n";
-    llvm_unreachable("triggering standard assertion failure routine");
+    llvm::report_fatal_error("triggering standard assertion failure routine");
   }
 };