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

Summary: This allows to remove `using namespace llvm;` in those *.cpp files

When we want to revisit the decision (everything resides in llvm::mca::*) in the future, we can move things to a nested namespace of llvm::mca::, to conceptually make them separate from the rest of llvm::mca::*

Reviewers: andreadb, mattd

Reviewed By: andreadb

Subscribers: javed.absar, tschuett, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D53407

llvm-svn: 345612
diff --git a/llvm/tools/llvm-mca/lib/Pipeline.cpp b/llvm/tools/llvm-mca/lib/Pipeline.cpp
index ad49522..309f415 100644
--- a/llvm/tools/llvm-mca/lib/Pipeline.cpp
+++ b/llvm/tools/llvm-mca/lib/Pipeline.cpp
@@ -17,12 +17,11 @@
 #include "HWEventListener.h"
 #include "llvm/Support/Debug.h"
 
+namespace llvm {
 namespace mca {
 
 #define DEBUG_TYPE "llvm-mca"
 
-using namespace llvm;
-
 void Pipeline::addEventListener(HWEventListener *Listener) {
   if (Listener)
     Listeners.insert(Listener);
@@ -95,3 +94,4 @@
     Listener->onCycleEnd();
 }
 } // namespace mca.
+} // namespace llvm