There is an issues with LLVM versions >=13 on SLES15 SP5. None of the Package
Hub versions can be installed because the Clang-tool dependency is missing. For
example, LLVM for SLES15 SP4 includes Clang-tool, but for SP5 it doesn't:
https://packagehub.suse.com/packages/llvm13/

The same issue applies to LLVM 14. Here's a brief log example:
# grep PRETTY /etc/os-release
PRETTY_NAME="SUSE Linux Enterprise Server 15 SP5"
# zypper install clang14-devel
Refreshing service 'Basesystem_Module_15_SP5_x86_64'.
Refreshing service 'Desktop_Applications_Module_15_SP5_x86_64'.
Refreshing service 'Development_Tools_Module_15_SP5_x86_64'.
Refreshing service 'Legacy_Module_15_SP5_x86_64'.
Refreshing service 'SUSE_Linux_Enterprise_Server_15_SP5_x86_64'.
Refreshing service 'SUSE_Package_Hub_15_SP5_x86_64'.
Refreshing service 'Server_Applications_Module_15_SP5_x86_64'.
Loading repository data...
Reading installed packages...
Resolving package dependencies...

Problem: nothing provides 'clang-tools >= 14.0.6' needed by the to be installed
clang14-devel-14.0.6-bp155.4.10.x86_64
 Solution 1: do not install clang14-devel-14.0.6-bp155.4.10.x86_64
 Solution 2: break clang14-devel-14.0.6-bp155.4.10.x86_64 by ignoring some of
its dependencies
# zypper info clang14-devel
Refreshing service 'Basesystem_Module_15_SP5_x86_64'.
Refreshing service 'Desktop_Applications_Module_15_SP5_x86_64'.
Refreshing service 'Development_Tools_Module_15_SP5_x86_64'.
Refreshing service 'Legacy_Module_15_SP5_x86_64'.
Refreshing service 'SUSE_Linux_Enterprise_Server_15_SP5_x86_64'.
Refreshing service 'SUSE_Package_Hub_15_SP5_x86_64'.
Refreshing service 'Server_Applications_Module_15_SP5_x86_64'.
Loading repository data...
Reading installed packages...

Information for package clang14-devel:
--------------------------------------
Repository     : SUSE-PackageHub-15-SP5-Backports-Pool
Name           : clang14-devel
Version        : 14.0.6-bp155.4.10
Arch           : x86_64
Vendor         : openSUSE
Support Level  : unknown
Installed Size : 17.3 MiB
Installed      : No
Status         : not installed
Source package : llvm14-14.0.6-bp155.4.10.src
Upstream URL   : https://www.llvm.org/
Summary        : CLANG frontend for LLVM (devel package)
Description    :
    This package contains the clang (C language) frontend for LLVM.
    (development files)
# zypper search clang | grep tools
#

At the same moment it's possible to install LLVM >=13 from OpenSUSE repos, for
example, adding this to repos config
"http://download.opensuse.org/distribution/leap/15.5/repo/oss/"