Check for invalid projects passed in LLVM_ENABLE_PROJECTS

This is catching misconfiguration. For example one of my automation
had a typo running `-DLLVM_ENABLE_PROJECTS=nlir` and it was just
silently ignored. Instead, an error will now be displayed.

Recommit 4121ac1e after fixing it to not fail on "all"

Differential Revision: https://reviews.llvm.org/D110016