clang fails to find C++ headers in presence of gcc-12 package

[Ratchanan Srirattanamet]

This is a weird one. On a clean rootfs (e.g. with `docker run -it --rm ubuntu:22.04`), clang can find C++ headers just fine.

```
# apt update; apt install --no-install-recommends clang
<snip>
# echo '#include <iostream>' | clang -x c++ -E - >/dev/null
#
```

But as soon as gcc-12 is also installed on the system (which can be pulled in by e.g. dkms), the exact same "source" doesn't pre-process anymore.

```
# apt install --no-install-recommends gcc-12
<snip>
# echo '#include <iostream>' | clang -x c++ -E - >/dev/null
<stdin>:1:10: fatal error: 'iostream' file not found
#include <iostream>
         ^~~~~~~~~~
1 error generated.
#
```

Upon further inspection, it seems like it just looks for headers in non-existent path, and somehow has "12" in it (but the resolved path isn't?)

```
# echo '#include <iostream>' | clang -x c++ -E -Wp,-v - >/dev/null
clang -cc1 version 14.0.0 based upon LLVM 14.0.0 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/backward"
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../x86_64-linux-gnu/include"
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/bin/../lib/gcc/x86_64-linux-gnu/12/../../../../include/c++
 /usr/lib/llvm-14/lib/clang/14.0.0/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
<stdin>:1:10: fatal error: 'iostream' file not found
#include <iostream>
         ^~~~~~~~~~
1 error generated.
```

It almost seems like clang tries to detect the highest installed version of GCC, but doesn't prepare for the fact that g++ 12 isn't installed. So, I'm not sure who's at fault here...

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: clang-14 1:14.0.0-1ubuntu1
Uname: Linux 6.1.11-gitv6.1.11-1-gb07abe8e29e1 x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu82.3
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: ubuntu:GNOME
Date: Sat Mar 4 01:17:58 2023
InstallationDate: Installed on 2021-03-15 (718 days ago)
InstallationMedia: Ubuntu 20.04.2.0 LTS "Focal Fossa" - Release amd64 (20210209.1)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=th_TH.UTF-8
 SHELL=/bin/bash
RebootRequiredPkgs: Error: path contained symlinks.
SourcePackage: llvm-toolchain-14
UpgradeStatus: Upgraded to jammy on 2022-10-22 (132 days ago)

[Launchpad Janitor (janitor)]

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

[Tom (tom-sogetthis)]

The same happens on Ubuntu 20.04 with clang-10 and gcc-10 (when gcc-10 is installed in addition to default gcc-9).

[Tom (tom-sogetthis)]

This problem happens when you install "gcc-12" package.

If you install "g++-12" package instead, there is no problem.

"gcc-12" package does not install C++ standard library, while "g++-12" package does install it.