Currently, `ll_infer_type` is responsible for telling the user about
`noncomputable` when a definition depends on one without executable
code. However, this is imperfect because type inference does not check
every subexpression. This leads to errors later on that users find to be
hard to interpret.
Now, `Lean.IR.checkDecls` has a friendlier error message when it
encounters constants without compiled definitions, suggesting to
consider using `noncomputable`. While this function is an internal IR
consistency check, it is also reasonable to have it give an informative
error message in this particular case. The suggestion to use
`noncomputable` is limited to just unknown constants.
Some alternatives would be to either (1) create another checker just for
missing constants, (2) change `ll_infer_type` to always visit every
subexpression no matter if they are necessary for inferring the type, or
(3) investigate whether `tests/lean/run/1785.lean` is due to a deeper
issue.
Closes#1785
