Revert part of r237975, "Fix Clang -Wmissing-override warning" in DIContext.h, to appease g++-4.7.

  llvm/include/llvm/DebugInfo/DIContext.h:144:11: error:   overriding ‘virtual llvm::LoadedObjectInfo::~LoadedObjectInfo() noexcept (true)’

It seems the destructor in the base class may not be "default".

llvm-svn: 238000