Ghci environment: Do not remove shadowed ids

Names defined earier but shadowed need to be kept around, e.g. for type
signatures:
```
ghci> data T = T
ghci> let t = T
ghci> data T = T
ghci> :t t
t :: Ghci1.T
```
and indeed they can be used:
```
ghci> let t2 = Ghci1.T :: Ghci1.T
ghci> :t t2
t2 :: Ghci1.T
```

However, previously this did not happen for ids (non-types), although they
are still around under the qualified name internally:
```
ghci> let t = "other t"
ghci> t'

<interactive>:8:1: error:
    • Variable not in scope: t'
    • Perhaps you meant one of these:
        ‘Ghci2.t’ (imported from Ghci2), ‘t’ (line 7), ‘t2’ (line 5)
ghci> Ghci2.t

<interactive>:9:1: error:
    • GHC internal error: ‘Ghci2.t’ is not in scope during type checking, but it passed the renamer
      tcl_env of environment: []
    • In the expression: Ghci2.t
      In an equation for ‘it’: it = Ghci2.t
```

This fixes the problem by simply removing the code that tries to remove
shadowed ids from the environment. Now you can refer to shadowed ids using
`Ghci2.t`, just like you can do for data and type constructors. This
simplifies the code, makes terms and types more similar, and also
fixes #20455.

Now all names ever defined in GHCi are in `ic_tythings`, which is printed by
`:show bindings`. But for that commands, it seems to be more ergonomic
to only list those bindings that are not shadowed. Or, even if it is not
more ergonomic, it’s the current behavour. So let's restore that by filtering
in `icInScopeTTs`.

Of course a single `TyThing` can be associated with many names. We keep
it it in the bindings if _any_ of its names are still visible
unqualifiedly. It's a judgement call.

This commit also turns a rather old comment into a test files.
The comment is is rather stale and things are better explained
elsewhere. Fixes #925.

Two test cases are regressing:

                   T14052(ghci) ghc/alloc  2749444288.0 12192109912.0 +343.4% BAD
               T14052Type(ghci) ghc/alloc  7365784616.0 10767078344.0  +46.2% BAD

This is not unexpected; the `ic_tythings list grows` a lot more if we
don’t remove shadowed Ids. I tried to alleviate it a bit with earlier
MRs, but couldn’t make up for it completely.

Metric Increase:
    T14052
    T14052Type

1 files changed, 40 insertions, 0 deletions

diff --git a/docs/users_guide/ghci.rst b/docs/users_guide/ghci.rst
index 72fba318bf..07377dab77 100644
--- a/docs/users_guide/ghci.rst
+++ b/docs/users_guide/ghci.rst
@@ -910,6 +910,46 @@ GHCi knows about. Using :ghci-cmd:`:module` or ``import`` to try bring into
 scope a non-loaded module may result in the message
 ``module M is not loaded``.
 
+Shadowing and the ``Ghci1`` module name
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Bindings on the prompt can shadow earlier bindings:
+
+.. code-block:: none
+
+    ghci> let foo = True
+    ghci> let foo = False
+    ghci> :show bindings
+    foo :: Bool = False
+
+But the shadowed thing still exists, and may show up again later, for example
+in a type signature:
+
+.. code-block:: none
+
+    ghci> data T = A | B deriving Eq
+    ghci> let a = A
+    ghci> data T = ANewType
+    ghci> :t a
+    a :: Ghci1.T
+
+Now the type of ``a`` is printed using the fully qualified name of ``T``, using
+the module name ``Ghci1`` (and ``Ghci2`` for the next set of bindings, and so
+on). You can use these qualified names as well:
+
+.. code-block:: none
+
+    ghci> a == Ghci1.A
+    True
+    ghci> let a = False -- shadowing a
+    ghci> Ghci2.a == Ghci1.A
+    True
+
+The command ``:show bindings`` only shows bindings that are not shadowed.
+Bindings that define multiple names, such as a type constructor and its data
+constructors, are shown if *any* defined name is still available without the
+need for qualification.
+
 The ``it`` variable
 ~~~~~~~~~~~~~~~~~~~