Add no-op flag to enable mutable byte arrays (#3934)

This PR adds a flag that we will propagate to downstream semantics to
explicitly opt them in to the _current_ behaviour of K w.r.t. the
`Bytes` sort. It is currently a no-op, but will eventually provide a
potential performance increase by introducing potentially unsound
behaviour.

llvm-backend/src/main/java/org/kframework/backend/llvm/LLVMKompileOptions.java

@@ -106,4 +106,10 @@ public List<String> convert(String str) {
       description = "Emit proof hint instrumentation code into the generated interpreter",
       hidden = true)
   public boolean enableProofHints;
+
+  @Parameter(
+      names = "--llvm-mutable-bytes",
+      description = "Use a faster, unsound representation for byte arrays on the LLVM backend",
+      hidden = true)
+  public boolean llvmMutableBytes;
 }