Fixed LLVM search.

[bjoo]

OK it turns out LLVM's LLVMConfigVersion.cmake seems to not support range based searches. This PR gets around this by finding any available LLVM and then just comparing the major version.

The problem: suppose we want to find LLVM version X.Y.Z or more recent and we issue
find_package(LLVM X.Y.Z). We have LLVM version P.Q.R on the CMAKE prefix Path.
The find_package() will only succeed if: X=P, Y=Z and R>=Z. It will not succeed for a range or a minor version difference. This is strict and it cannot acommodate a change going e.g. to the top of the release-20.x branch when asking for version 20.

The workaround here is to just ask to find any LLVM and check the major number post-facto:

find_package(LLVM CONFIG REQUIRED) # Generic find
if( ${LLVM_VERSION_MAJOR} EQUAL ... ) # We can process just major version equality