This change was provoked by Trac #10720, where a missing
INLINEABLE pragma gave very poor performance.

The change is to warn when an imported function is not specialised
in a situation where the user expects it to be.

New flags
  -fwarn-missed-specialisations
  -fwarn-all-missed-specialisations

Documented in user manual.
See Note [Warning about missed specialisations]