[SampleFDO] Provide a virtual desructor for SampleProfileLoaderBaseImpl

This patch fixes a warning:

  llvm-project/llvm/include/llvm/ProfileData/SampleProfileLoaderBaseImpl.h:69:7:
  error: 'llvm::SampleProfileLoaderBaseImpl' has virtual functions but
  non-virtual destructor [-Werror,-Wnon-virtual-dtor]

Differential Revision: https://reviews.llvm.org/D96810