Duplicated getLLVMBuildFactory and adapted it to cmake. The original should be
removed once autoconf is no longer supported.
Drop the following arguments from cmake version since nobody uses them anymore:
- llvmgccdir
- valgrind
- examples
- expensive_checks
- enable_targets
In the case of valgrind, 'llvm-x86_64-linux-vg_leak' references it but this
builder is disabled.