Promote Enzyme pass remarks to warnings when they indicate result may be incorrect

[btalamini]

I have a case in which a derivative is silently computed incorrectly. If I compile my code with the flag -Rpass=enzyme, the following remark is generated:

/Users/talamini1/dev/mfem/linalg/tensor.hpp:83:70: remark: Mismatched activity for: ret ptr %5, !dbg !3541 const val: %5 = getelementptr inbounds [3 x %"struct.mfem::future::tensor.0"], ptr %3, i64 0, i64 %4, !dbg !3540 [-Rpass=enzyme]
   83 |    MFEM_HOST_DEVICE const tensor< T, n1 >& operator[](int i) const { return values[i]; }
      |                                                                      ^

@wsmoses diagnosed the problem and has given me a workaround for my specific code (thanks!). The more general issue here is that in cases like this, where correctness cannot be guaranteed, compile-time warnings should be generated.

[eskayML]

looks like the enzyme pass is not properly handling cases where correctness cannot be guaranteed. i'm guessing we can modify the pass to emit warnings in these situations by checking for certain conditions in the IR. should i open a PR for this?