[llvm-branch-commits] [CI] Explicitly compute needed runtime targets (PR #142695)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/142695

This patch adjusts the compute_projects script to explicitly determine
what runtimes should be built and what runtimes should be tested. This
is mainly to support enabling runtimes for LLDB testing but not test
them unless we are building clang.