[flang] [llvm] [flang] Register and lower SECNDS (stubbed implementation) (PR #151878)

[Eugene Epshteyn via llvm-commits]

eugeneepshteyn wrote:

@mlir-maiden , I submitted runtime implementation for `secnds()` upstream. You can now test end-to-end with trying to make the actual function call.

In my implementation I defined runtime wrapper function for `secnds()` as follows:
```
float RTNAME(Secnds)(float *refTime, const char *sourceFile, int line) {
  Terminator terminator{sourceFile, line};
  RUNTIME_CHECK(terminator, refTime != nullptr);
  return FORTRAN_PROCEDURE_NAME(secnds)(refTime);
}
```
Feel free to modify, if that doesn't work for you.

Peter also asked me to make the core implementation of `secnds` runtime into a template, so that it could be instantiated with `double` in case we want to also add `DSECNDS()`. (If you end up implementing `DSECNDS()`, I recommend you do that as a separate PR.)

https://github.com/llvm/llvm-project/pull/151878