[SampleFDO] Fix MSVC "namespace uses itself" warning (NFC)

MSVC warning:
```
SampleProfileLoaderBaseImpl.h(41): warning C4515: 'llvm': namespace uses itself
```

Author: nullptr-cpp

llvm/include/llvm/Transforms/Utils/SampleProfileLoaderBaseImpl.h

@@ -38,7 +38,6 @@
 #include "llvm/Transforms/Utils/SampleProfileLoaderBaseUtil.h"
 
 namespace llvm {
-using namespace llvm;
 using namespace sampleprof;
 using namespace sampleprofutil;
 using ProfileCount = Function::ProfileCount;