[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/CodeRegion.h b/llvm/tools/llvm-mca/CodeRegion.h
index 21ca8da..6ca2bd1 100644
--- a/llvm/tools/llvm-mca/CodeRegion.h
+++ b/llvm/tools/llvm-mca/CodeRegion.h
@@ -41,6 +41,7 @@
 #include "llvm/Support/SourceMgr.h"
 #include <vector>
 
+namespace llvm {
 namespace mca {
 
 /// A region of assembly code.
@@ -123,5 +124,6 @@
 };
 
 } // namespace mca
+} // namespace llvm
 
 #endif