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".

stdlib/public/core/StaticString.swift

@@ -94,6 +94,11 @@ public struct StaticString
     return Int(_utf8CodeUnitCount)
   }
 
+  @_alwaysEmitIntoClient @_transparent
+  internal var unsafeRawPointer: Builtin.RawPointer {
+    return Builtin.inttoptr_Word(_startPtrOrData)
+  }
+
   /// A Boolean value indicating whether the static string stores a pointer to
   /// ASCII or UTF-8 code units.
   @_transparent