SIL: add a failure message operand to Builtin.condfail

The SIL generation for this builtin also changes: instead of generating the cond_fail instructions upfront, let the optimizer generate it, if the operand is a static string literal.
In worst case, if the second operand is not a static string literal, the Builtin.condfail is lowered at the end of the optimization pipeline with a default message: "unknown program error".

lib/SILOptimizer/Transforms/AccessEnforcementReleaseSinking.cpp

@@ -129,6 +129,7 @@ static bool isBarrier(SILInstruction *inst) {
     case BuiltinValueKind::GetObjCTypeEncoding:
     case BuiltinValueKind::Swift3ImplicitObjCEntrypoint:
     case BuiltinValueKind::WillThrow:
+    case BuiltinValueKind::CondFail:
     case BuiltinValueKind::PoundAssert:
     case BuiltinValueKind::GlobalStringTablePointer:
       return false;