[lldb][NFCI] Change logic to find clang resource dir in standalone builds

As of 0beffb85 there is a CMake
function to actually calculate the relative path to the clang resource
directory. Currently we have some bespoke logic that looks in a few
places, but with this new function we should be able to eliminate some
complexity here.

Also, I moved the functionality from LLDBConfig to LLDBStandalone since
it is only used in standalone builds.

Differential Revision: https://reviews.llvm.org/D156270