Revert "cmake/llvm: Don't unset LLVM_CONFIG_EXE"

This reverts commit 28476a5ee94d311319941b54e9da66210690ce70.

It also adds a comment to explain the purpose of this line, to prevent
this mistake from being repeated.
793f820b39Andrew Kelley committed on 5/25/2024, 4:04:52 PMparent0b0625c
1 file changedLine totals unavailable