[StreamExecutor] Add KernelLoaderSpec

Summary:
Add definitions for the KernelLoaderSpec and MultiKernelLoaderSpec
classes to StreamExecutor. Instances of these classes are generated by the
compiler in order to provide host code with a handle to device code.

Reviewers: jlebar, tra

Subscribers: parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D23038

llvm-svn: 277615