[runtimes][NFC] Colocate handling of LLVM_ENABLE_PROJECTS and LLVM_ENABLE_RUNTIMES

This will make the following patches to migrate projects off of the
LLVM_ENABLE_PROJECTS build onto the LLVM_ENABLE_RUNTIMES build much
easier to comprehend. This patch should be a NFC since it keeps the
same set of runtimes being built by default.

Differential Revision: https://reviews.llvm.org/D132478