Improve aggressive specialisation

This patch fixes #21286, by not unboxing dictionaries in
worker/wrapper (ever). The main payload is tiny:

* In `GHC.Core.Opt.DmdAnal.finaliseArgBoxities`, do not unbox
  dictionaries in `get_dmd`.  See Note [Do not unbox class dictionaries]
  in that module

* I also found that imported wrappers were being fruitlessly
  specialised, so I fixed that too, in canSpecImport.
  See Note [Specialising imported functions] point (2).

In doing due diligence in the testsuite I fixed a number of
other things:

* Improve Note [Specialising unfoldings] in GHC.Core.Unfold.Make,
  and Note [Inline specialisations] in GHC.Core.Opt.Specialise,
  and remove duplication between the two. The new Note describes
  how we specialise functions with an INLINABLE pragma.

  And simplify the defn of `spec_unf` in `GHC.Core.Opt.Specialise.specCalls`.

* Improve Note [Worker/wrapper for INLINABLE functions] in
  GHC.Core.Opt.WorkWrap.

  And (critially) make an actual change which is to propagate the
  user-written pragma from the original function to the wrapper; see
  `mkStrWrapperInlinePrag`.

* Write new Note [Specialising imported functions] in
  GHC.Core.Opt.Specialise

All this has a big effect on some compile times. This is
compiler/perf, showing only changes over 1%:

Metrics: compile_time/bytes allocated
-------------------------------------
                LargeRecord(normal)  -50.2% GOOD
           ManyConstructors(normal)   +1.0%
MultiLayerModulesTH_OneShot(normal)   +2.6%
                  PmSeriesG(normal)   -1.1%
                     T10547(normal)   -1.2%
                     T11195(normal)   -1.2%
                     T11276(normal)   -1.0%
                    T11303b(normal)   -1.6%
                     T11545(normal)   -1.4%
                     T11822(normal)   -1.3%
                     T12150(optasm)   -1.0%
                     T12234(optasm)   -1.2%
                     T13056(optasm)   -9.3% GOOD
                     T13253(normal)   -3.8% GOOD
                     T15164(normal)   -3.6% GOOD
                     T16190(normal)   -2.1%
                     T16577(normal)   -2.8% GOOD
                     T16875(normal)   -1.6%
                     T17836(normal)   +2.2%
                    T17977b(normal)   -1.0%
                     T18223(normal)  -33.3% GOOD
                     T18282(normal)   -3.4% GOOD
                     T18304(normal)   -1.4%
                    T18698a(normal)   -1.4% GOOD
                    T18698b(normal)   -1.3% GOOD
                     T19695(normal)   -2.5% GOOD
                      T5837(normal)   -2.3%
                      T9630(normal)  -33.0% GOOD
                      WWRec(normal)   -9.7% GOOD
             hard_hole_fits(normal)   -2.1% GOOD
                     hie002(normal)   +1.6%

                          geo. mean   -2.2%
                          minimum    -50.2%
                          maximum     +2.6%

I diligently investigated some of the big drops.

* Caused by not doing w/w for dictionaries:
    T13056, T15164, WWRec, T18223

* Caused by not fruitlessly specialising wrappers
    LargeRecord, T9630

For runtimes, here is perf/should+_run:

Metrics: runtime/bytes allocated
--------------------------------
               T12990(normal)   -3.8%
                T5205(normal)   -1.3%
                T9203(normal)  -10.7% GOOD
        haddock.Cabal(normal)   +0.1%
         haddock.base(normal)   -1.1%
     haddock.compiler(normal)   -0.3%
        lazy-bs-alloc(normal)   -0.2%
------------------------------------------
                    geo. mean   -0.3%
                    minimum    -10.7%
                    maximum     +0.1%

I did not investigate exactly what happens in T9203.

Nofib is a wash:

+-------------------------------++--+-----------+-----------+
|                               ||  | tsv (rel) | std. err. |
+===============================++==+===========+===========+
|                     real/anna ||  |    -0.13% |      0.0% |
|                      real/fem ||  |    +0.13% |      0.0% |
|                   real/fulsom ||  |    -0.16% |      0.0% |
|                     real/lift ||  |    -1.55% |      0.0% |
|                  real/reptile ||  |    -0.11% |      0.0% |
|                  real/smallpt ||  |    +0.51% |      0.0% |
|          spectral/constraints ||  |    +0.20% |      0.0% |
|               spectral/dom-lt ||  |    +1.80% |      0.0% |
|               spectral/expert ||  |    +0.33% |      0.0% |
+===============================++==+===========+===========+
|                     geom mean ||  |           |           |
+-------------------------------++--+-----------+-----------+

I spent quite some time investigating dom-lt, but it's pretty
complicated.  See my note on !7847.  Conclusion: it's just a delicate
inlining interaction, and we have plenty of those.

Metric Decrease:
    LargeRecord
    T13056
    T13253
    T15164
    T16577
    T18223
    T18282
    T18698a
    T18698b
    T19695
    T9630
    WWRec
    hard_hole_fits
    T9203

0 files changed, 0 insertions, 0 deletions