There are multiple reasons to do this:

• unable to pin compiler to specific version, since apt.llvm.org contains only latest minor version for each major
• --target mismatch - if --target does not match LLVM_DEFAULT_TARGET_TRIPLE then clang will not find runtime sanitizers libraries (but right now there is a workaround in docker build images)
• patching

Refs:

• [RFC] [clang][Toolchain] Treat "pc"/"unknown" vendor interchangeable llvm/llvm-project#97802
• Sanitizers builds are broken #66049

Cc: @alesapin