[MachineVerifier] Drop a spurious const

As of r298987 the argument is a value that we std::move, so it
shouldn't be const anymore.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298999 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineVerifier.cpp

@@ -260,7 +260,7 @@ namespace {
     static char ID; // Pass ID, replacement for typeid
     const std::string Banner;
 
-    MachineVerifierPass(const std::string banner = std::string())
+    MachineVerifierPass(std::string banner = std::string())
       : MachineFunctionPass(ID), Banner(std::move(banner)) {
         initializeMachineVerifierPassPass(*PassRegistry::getPassRegistry());
       }