[SE] Host platform implementation

Summary:
This implementation does not currently support multiple concurrent streams, and
it won't allow kernels to be launched with grids larger than one block or
blocks larger than one thread. These limitations could be removed in the future
by launching new threads on the host, but that is not done in this
implementation.

Reviewers: jlebar

Subscribers: beanz, mgorny, jprice, parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24473

llvm-svn: 281377