Revert "[libclc] build libclc as LLVM runtime" (#21407)
Reverts intel/llvm#21221
libclc-remangler links against Clang/LLVM libraries built with the host
compiler, but the tool itself is built with the just-built Clang. This
causes an ABI issue around std::optional parameter in
MemoryBuffer::getFileOrSTDIN.
Jira: CMPLRLLVM-73868
