[Target] Introduce Process::GetLanguageRuntimes
Currently there's not really a good way to iterate over the language runtimes a
process has. This is sometimes desirable (as seen in my change to Thread).
Additionally, there's not really a good reason to iterate over every available
language, but rather only over languages for which we have a plugin loaded.
Reviewers: JDevlieghere, davide, jingham
Differential Revision: https://reviews.llvm.org/D62562