cmake: Print all LLVM config errors instead of just the last one

[sin-ack]

If you have multiple llvm-config executables in your path, and all of them cause failures, then only the last failure will be printed. This can cause confusion when the multiple llvm-config executables are from different major LLVM versions, i.e. LLVM 13 and 14, which might mask an error that happened on the LLVM 14 llvm-config with an unrelated error.

This commit makes it so that all errors are collected into a list and printed all at once; this way, you can see how each llvm-config executable failed to configure properly. Note that the failures still won't be printed if a successful configuration is found.