As dicussed in pgsql-general on May 28th there are package conflicts when updating postgresql12 with postgis30: ... at the moment we are facing similar conflicts on Oracle LInux 7 (wich is derived from RHEL) – we manage our machines using Spacewalk. The conflicts occur (as expected) on Spacewalk as well as on manually using yum:
llvm-toolset is needed by postgresql11-devel and postgresql12-devel in this setup it seems, but why do you need those? If what you want is Postgis, then the postgis RPM should be enough and that shouldn't need devel packages at all, and therefore not the llvm packages.
If you do actually need the devel packages, you need to enable the scl repo. On centos, that means install centos-release-scl. On RHEL, I believe you do "yum-config-manager --enable rhel-server-rhscl-7-rpms".
But again,if "all" you want is PostGIS, they shouldn't be needed.