[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/Assembler.cpp b/llvm/tools/llvm-exegesis/lib/Assembler.cpp
index 1576cfe..771a6e9 100644
--- a/llvm/tools/llvm-exegesis/lib/Assembler.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Assembler.cpp
@@ -23,6 +23,7 @@
 #include "llvm/MC/MCInstrInfo.h"
 #include "llvm/Support/MemoryBuffer.h"
 
+namespace llvm {
 namespace exegesis {
 
 static constexpr const char ModuleID[] = "ExegesisInfoTest";
@@ -295,3 +296,4 @@
 }
 
 } // namespace exegesis
+} // namespace llvm