Put RdrName in the foExt field of FieldOcc

The main purpose of this commit is to rip RdrName out of FieldOcc, in
accordance with #21592, and as a side note it has simplified the method
we use to deal with ambiguity somewhat.

To do the first, we make FieldOccs store (LIdP p) instead of always
storing Located RdrName, and moved the readername to the extension
points where necessary.

For the second, well, we just turn an ambiguous RdrName into a unbound
Name through mkUnboundName. Later during disambiguateRecordBinds of the
type checking phase, we will try and do type-directed disambiguation based
on the rdrName field (for now), so this hack works out fine.

See Note [Ambiguous FieldOcc in record updates] for more details.

There are two additional minor changes in this commit:
*  The HsRecSel constructor of HsExpr has been moved to the extension
constuctors, since its really GHC specific.
*  HsProjection no longer has a Located DotFieldOcc as a field, but just a
regular DotFieldOcc, since DotFieldOcc already wraps a located
FieldLabelString

co-authored by: @Jade       <Jade512@proton.me>
                @alt-romes  <rodrigo.m.mesquita@gmail.com>

(cherry picked from commit 1587cccf)