Qt Creator uses Clang-8 and expects Clang-8 include files but has no dependency on clang-8 include files

[M W]

```
lsb_release -rd
Description: Ubuntu 20.04.1 LTS
Release: 20.04
```

qtcreator version:

```
apt show qtcreator
Package: qtcreator
Version: 4.11.0-2build2
Priority: optional
Section: universe/devel
Origin: Ubuntu
Maintainer: Ubuntu Developers <email address hidden>
Original-Maintainer: Debian Qt/KDE Maintainers <email address hidden>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 115 MB
Depends: libqt5sql5-sqlite, qml-module-qtqml-models2, qml-module-qtquick-controls (>= 5.5.0~), qml-module-qtquick2 (>= 5.5.0~), qtchooser, qtcreator-data (= 4.11.0-2build2), libc6 (>= 2.29), libclang1-8 (>= 1:7~svn334604-1~+0~), libgcc-s1 (>= 3.0), libkf5syntaxhighlighting5, libllvm8 (>= 1:8~svn298832-1~), libqbscore1.13 (>= 1.12.0), libqt5concurrent5 (>= 5.6.2~), libqt5core5a (>= 5.12.2), libqt5designer5 (>= 5.6.2~), libqt5designercomponents5 (>= 5.6.2~), libqt5gui5 (>= 5.12.2) | libqt5gui5-gles (>= 5.12.2), libqt5help5 (>= 5.9.0), libqt5network5 (>= 5.6.2~), libqt5printsupport5 (>= 5.6.2~), libqt5qml5 (>= 5.2.0~rc1), libqt5quick5 (>= 5.9.0~beta) | libqt5quick5-gles (>= 5.9.0~beta), libqt5quickwidgets5 (>= 5.11.0), libqt5script5 (>= 5.6.2~), libqt5serialport5 (>= 5.9.0), libqt5sql5 (>= 5.6.2~), libqt5widgets5 (>= 5.12.2), libqt5xml5 (>= 5.6.2~), libstdc++6 (>= 9), qtbase-abi-5-12-8, qtdeclarative-abi-5-12-8
Recommends: clang, clang-tidy, gdb, make, qmlscene, qt5-doc, qt5-qmltooling-plugins, qtbase5-dev-tools, qtcreator-doc, qtdeclarative5-dev-tools, qttools5-dev-tools, qttranslations5-l10n, qtxmlpatterns5-dev-tools, xterm | x-terminal-emulator
Suggests: clazy (>= 1.5), cmake, g++, git, kate-data, subversion, valgrind
Breaks: qtcreator-data (<< 4.5.2-2~)
Replaces: qtcreator-data (<< 4.5.2-2~)
Homepage: https://doc.qt.io/qt-5/topics-app-development.html
Download-Size: 29.0 MB
APT-Manual-Installed: yes
APT-Sources: http://gb.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
Description: integrated development environment (IDE) for Qt
 Qt Creator is a cross-platform integrated development environment (IDE)
 designed to make development with the Qt application framework faster
 and easier.
 .
 It includes:
  * An advanced C++ code editor
  * Integrated GUI layout and forms designer
  * Project and build management tools
  * Integrated, context-sensitive help system
  * Visual debugger
  * Rapid code navigation tools
  * Supports multiple platforms
  * Qt Quick Designer
```

Qt Creator has a code model which shows errors as the project is being worked on.

However, in the event that clang-8's include files are missing (which was the case because I did not have clang-8 installed, just libclang1-8 which was the actual dependency, and clang-10), the Clang code model shows errors about "stddef" not being found, thus indirectly causing C++ Qt projects to show a lot of phantom errors.

This is essentially a missing-dependency issue; Qt Creator expects the include files of the Clang version it was compiled with, and it doesn't get them.

The exact issue was determined via:
```
ldd /usr/lib/x86_64-linux-gnu/qtcreator/libexec/clangbackend
 linux-vdso.so.1 (0x00007ffde3bee000)
 libSqlite.so.4 => /usr/lib/x86_64-linux-gnu/qtcreator/libexec/../libSqlite.so.4 (0x00007fe611caf000)
 libClangsupport.so.4 => /usr/lib/x86_64-linux-gnu/qtcreator/libexec/../libClangsupport.so.4 (0x00007fe611c2d000)
 libUtils.so.4 => /usr/lib/x86_64-linux-gnu/qtcreator/libexec/../libUtils.so.4 (0x00007fe61196d000)
 libclang-8.so.1 => /lib/x86_64-linux-gnu/libclang-8.so.1 (0x00007fe60ffc2000)
<...further details omitted...>
```

Installing package "clang-8" completely resolved the issue.
If that is not an available workaround, providing /usr/include/clang/8/include (symlink to other Clang include directory?) should be sufficient, but I haven't tested this.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: qtcreator 4.11.0-2build2
ProcVersionSignature: Ubuntu 5.4.0-58.64-generic 5.4.73
Uname: Linux 5.4.0-58-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.14
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: MATE
Date: Thu Jan 7 14:20:01 2021
InstallationDate: Installed on 2020-10-06 (93 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
SourcePackage: qtcreator
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Kristian (kmk)]

Also present in 20.04.4 LTS. Installing clang-8 from apt fixes code model.