[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
diff --git a/llvm/tools/llvm-exegesis/lib/Target.cpp b/llvm/tools/llvm-exegesis/lib/Target.cpp
index 8baa849..b7828a1 100644
--- a/llvm/tools/llvm-exegesis/lib/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Target.cpp
@@ -11,6 +11,7 @@
 #include "Latency.h"
 #include "Uops.h"
 
+namespace llvm {
 namespace exegesis {
 
 ExegesisTarget::~ExegesisTarget() {} // anchor.
@@ -109,3 +110,4 @@
 }
 
 } // namespace exegesis
+} // namespace llvm