An attempt to abandon omptarget out-of-tree builds.

I want to start using LLVM component libraries in libomptarget
to stop duplicating implementations already available in LLVM
(e.g. LLVMObject, LLVMSupport, etc.). Without relying on LLVM
in all libomptarget builds one has to provide fallback implementation
for each used LLVM feature.

This is an attempt to stop supporting out-of-llvm-tree builds of libomptarget.

I understand that I may need to revert this,
if this affects downstream projects in a bad way.

Differential Revision: https://reviews.llvm.org/D101509