Rodrigo Mesquita authored Mar 13, 2023 and Marge Bot committed May 04, 2023

Previously, the unit-id of ghc-the-library was fixed as `ghc`.
This was done primarily because the compiler must know the unit-id of
some packages (including ghc) a-priori to define wired-in names.

However, as seen in #20742, a reinstallable `ghc` whose unit-id is fixed
to `ghc` might result in subtle bugs when different ghc's interact.

A good example of this is having GHC_A load a plugin compiled by GHC_B,
where GHC_A and GHC_B are linked to ghc-libraries that are ABI
incompatible. Without a distinction between the unit-id of the ghc library
GHC_A is linked against and the ghc library the plugin it is loading was
compiled against, we can't check compatibility.

This patch gives a slightly better unit-id to ghc (ghc-version) by
(1) Not setting -this-unit-id to ghc, but rather to the new unit-id (modulo stage0)
(2) Adding a definition to `GHC.Settings.Config` whose value is the new unit-id.
    (2.1) `GHC.Settings.Config` is generated by Hadrian
    (2.2) and also by cab...