[StreamExecutor] Rename StreamExecutor to Executor Summary: No functional changes just renaming this class for better readability. Reviewers: jlebar Subscribers: jprice, parallel_libs-commits Differential Revision: https://reviews.llvm.org/D23574 llvm-svn: 278833 GitOrigin-RevId: a91dc70b18dd129761b66c1b76cbe64f54cd2eb1