[Clang][AMDGPU] Set LTO CG opt level based on Clang option

For AMDGCN default to mapping --lto-O# to --lto-CGO# in a 1:1 manner
(i.e. clang -O<N> implies --lto-O<N> and --lto-CGO<N>).

Ensure there is a means to override this via -Xoffload-linker and begin
to claim these arguments to avoid incorrect warnings that they are not
used.

Reviewed By: yaxunl, MaskRay

Differential Revision: https://reviews.llvm.org/D142499