[StreamExecutor] Clean up device copy comments

Summary:
Consolidate Executor::synchronousCopy* and Stream::thenCopy* methods into
Doxygen method groups and combine all their comments into one section.

Also a "doc" target to the build files to use Doxygen to build the
documentation.

Reviewers: jlebar

Subscribers: jprice, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D23845

llvm-svn: 279654