[patch] Use a reasonable fallback for LLVM_CONF

In #19 I wrote that I couldn't compile the code via

  $ cmake -B build -S cgrep/ -DUSE_MONOLITH_LIBTOOLING=ON -DCMAKE_CXX_COMPILER=clang++
  $ cmake --build build

This is because I didn't set LLVM_CONF and it falls back to
llvm-config-10. There are two problems with this:

First, CMake didn't actually check if LLVM_CONF existed. I now check it
via the REQUIRED argument in find_program.

Second, my OS (Arch Linux) doesn't name the binary like llvm-config-10
but it's called llvm-config, so the fallback never works on Arch. Thus,
I added a non-complete but somewhat reasonable list of possible names.

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