[SPIR-V] Add linking using spirv-link.

Add support of linking files compiled into SPIR-V objects
using spirv-link.

Command line inteface examples:

clang --target=spirv64 test1.cl test2.cl

clang  --target=spirv64 test1.cl -o test1.o
clang  --target=spirv64 test1.o test2.cl -o test_app.out

This works independently from the SPIR-V generation method
(via an external tool or an internal backend) and applies
to either approach that is being used.

Differential Revision: https://reviews.llvm.org/D116266