Fix #14628: Panic (No skolem Info) in GHCi

This patch implements the [sugggestion from Simon (PJ)](#14628 (comment 146559)):
- Make `TcErrors.getSkolemInfo` return a `SkolemInfo` rather than an `Implication`.
- If `getSkolemInfo` gets `RuntimeUnk`s, just return a new data constructor in `SkolemInfo`, called `RuntimeUnkSkol`.
- In `TcErrors.pprSkols` print something sensible for a `RuntimeUnkSkol`.

The `getSkolemInfo` function paniced while formating suggestions to add type annotations (subfunction `suggestAddSig`)
to a *"Couldn't match type ‘x’ with ‘y’"* error message.
The `getSkolemInfo` function didn't find any Implication value and paniced.
With this patch the `getSkolemInfo` function does no longer panic, if it finds `RuntimeUnkSkol`s.

As the panic occured while processing an error message, we don't need to implement any new error message!
22 jobs for master in 434 minutes and 28 seconds (queued for 21 seconds)
Status Job ID Name Coverage
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  Full Build
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validate-aarch64-linux-deb9

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x86_64-linux
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x86_64-darwin
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Name Stage Failure
failed
pages Deploy
Synchronizing submodule url for 'utils/hsc2hs'
$ git submodule update --init --recursive
$ git checkout .gitmodules
$ git fetch https://gitlab.haskell.org/ghc/ghc-performance-notes.git refs/notes/perf:refs/notes/perf || true
$ mkdir -p public/doc
$ tar -xf haddock.html.tar.xz -C public/doc
tar: haddock.html.tar.xz: Cannot open: No such file or directory
tar: Error is not recoverable: exiting now
ERROR: Job failed: exit code 1
failed
doc-tarball Packaging
Synchronizing submodule url for 'utils/hsc2hs'
$ git submodule update --init --recursive
$ git checkout .gitmodules
$ git fetch https://gitlab.haskell.org/ghc/ghc-performance-notes.git refs/notes/perf:refs/notes/perf || true
From https://gitlab.haskell.org/ghc/ghc-performance-notes
f9223686b6..6b919d218b refs/notes/perf -> refs/notes/perf
$ if [ ! -f "$LINUX_BINDIST" ]; then # collapsed multi-line command
Error: ghc-x86_64-mingw32.tar.xz does not exist. Did the 64-bit Windows job fail?
ERROR: Job failed: exit code 1
failed
validate-aarch64-linux-deb9 Full Build
Uploading artifacts...
ghc-aarch64-linux-deb9.tar.xz: found 1 matching files
junit.xml: found 1 matching files

performance-metrics.tsv: found 1 matching files

Uploading artifacts to coordinator... ok
id=257678 responseStatus=201 Created token=bZcPeore
Uploading artifacts...
junit.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=257678 responseStatus=201 Created token=bZcPeore
ERROR: Job failed: exit code 1
failed
validate-x86_64-windows Full Build There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job log
failed
validate-x86_64-windows-hadrian Full Build
Runtime platform                                    arch=amd64 os=windows pid=10780 revision=1f513601 version=11.10.1
WARNING: ghc.tar.xz: no matching files
WARNING: junit.xml: no matching files
ERROR: No files to upload
Uploading artifacts...
Runtime platform
arch=amd64 os=windows pid=3520 revision=1f513601 version=11.10.1
WARNING: junit.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit status 1