[runtimes] Support sub-components in LLVM_RUNTIME_DISTRIBUTION_COMPONENTS

Right now, sub-components are only accounted for when the runtime has
been configured and the main LLVM configure is rerun after that. We can
make use of LLVM_RUNTIME_DISTRIBUTION_COMPONENTS to make sub-component
targets available from the start, but that currently clashes with the
sub-component target generation once they are picked up. Make sure we
don't create duplicate targets; an analogous check already exists in
runtime_default_target.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D138966