[llvm-exegesis] Move namespace exegesis inside llvm::

Summary:
This allows simplifying references of llvm::foo with foo when the needs
come in the future.

Reviewers: courbet, gchatelet

Reviewed By: gchatelet

Subscribers: javed.absar, tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D53455

llvm-svn: 344922