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.

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

GitOrigin-RevId: 4121ac1e7f96d826b793bd7e29c181f1eb8168bf
1 file changed