[runtimes] Handle interface targets in runtimes distribution components

As reported in [1], cxx-headers is not a valid distribution target
because it is an interface target in CMake. This breaks the most
basic MultiDistributionExample of the runtimes build.

This patch handles interface targets by getting rid of the assumption
that all distribution components have a target associated to them. It
is valid for a distribution component to only have a `install-FOO`
target.

In the case where only `cxx-headers` is provided as a distribution
component, `ninja toolchain-distribution` will not build anything
after this patch, since there is effectively nothing to build for
the `cxx-headers` target. However, running `ninja install-toolchain-distribution`
will build everything, as expected.

[1]: https://discord.com/channels/636084430946959380/636732894974312448/1010013287464828968

Differential Revision: https://reviews.llvm.org/D132387