[SYCL] Add proper handling of -fsycl-add-targets option

Up until now, multiple use cases did not function properly.
These can be divided into 2 groups:

1) multiple triples for different binaries not being propagated to
the final offload;

2) misbehaviour of -fsycl-targets and -fsycl-add-targets options'
conjunction, Driver not constructing jobs for the former.

Main steps taken:

1) -fsycl-add-targets option conjunction & syntax checking was
moved from SYCL's SpecializedBuilder to the Driver class to
unify its handling with -fsycl-targets & -fsycl-link-targets;

2) separate toolchains are now constructed for -fsycl-add-targets'
triples in order to conviniently propagate the triples to the offload
jobs;

3) default toolchain construction is now switched off by
-fsycl-add-targets, as is the case with other -fsycl*targets options;

4) clear differentiation between -fsycl-add-targets' AOT inputs
and other option's inputs (including AOT) at the SpecializedBuilder
level was added to create necessary jobs both for -fsycl-targets and
-fsycl-add-targets.

Signed-off-by: Artem Gindinson <[email protected]>

Author: Artem Gindinson