Make a helper function static. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212364 91177308-0d34-0410-b5e6-96231b3b80d8
MC: make MCSymbolData::dump work on const objects

This just lets us dump a const MCSymbolData object, no functionality
changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212365 91177308-0d34-0410-b5e6-96231b3b80d8