Create a LLVM_Util class (in llvm_util.{h,cpp}) that handles ALL direct calls
to LLVM APIs, and has NO special knowledge of OSL or its internals.

BackendLLVM (which is intimately tied to OSL) now is built on top of
LLVM_Util. Neither BackendLLVM nor the rest of liboslexec needs to directly
touch LLVM API calls and don't even need to include a single llvm header.

LLVM_Util has no dependencies on anything else in OSL (though does want
TypeDesc and ustring), and could be used as a standalone library from
other apps, to vastly simplify using LLVM to dynamically JIT code and
then execute it.  An example of this can be found in llvmutil_test.cpp.