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
Fangrui Song