Introduce SevIgnore Severity to suppress warnings

This commit introduces a new `Severity` type constructor called
`SevIgnore`, which can be used to classify diagnostic messages which are
not meant to be displayed to the user, for example suppressed warnings.

This extra constructor allows us to get rid of a bunch of redundant
checks when emitting diagnostics, typically in the form of the pattern:

```
when (optM Opt_XXX) $
  addDiagnosticTc (WarningWithFlag Opt_XXX) ...
```

Fair warning! Not all checks should be omitted/skipped, as evaluating some data
structures used to produce a diagnostic might still be expensive (e.g.
zonking, etc). Therefore, a case-by-case analysis must be conducted when
deciding if a check can be removed or not.

Last but not least, we remove the unnecessary `CmdLine.WarnReason` type, which is now
redundant with `DiagnosticReason`.

1 files changed, 1 insertions, 3 deletions

diff --git a/compiler/GHC/Tc/Gen/Bind.hs b/compiler/GHC/Tc/Gen/Bind.hs
index 228c3d3644..10294998c0 100644
--- a/compiler/GHC/Tc/Gen/Bind.hs
+++ b/compiler/GHC/Tc/Gen/Bind.hs
@@ -801,9 +801,7 @@ mkExport prag_fn insoluble qtvs theta
                   else addErrCtxtM (mk_impedance_match_msg mono_info sel_poly_ty poly_ty) $
                        tcSubTypeSigma sig_ctxt sel_poly_ty poly_ty
 
-        ; warn_missing_sigs <- woptM Opt_WarnMissingLocalSignatures
-        ; when warn_missing_sigs $
-              localSigWarn Opt_WarnMissingLocalSignatures poly_id mb_sig
+        ; localSigWarn Opt_WarnMissingLocalSignatures poly_id mb_sig
 
         ; return (ABE { abe_ext = noExtField
                       , abe_wrap = wrap