Add IDs and clone methods to NativeRawSymbol

All NativeRawSymbols will have a unique symbol ID (retrievable via
getSymIndexId).  For now, these are initialized to 0, but soon the
NativeSession will be responsible for creating the raw symbols, and it will
assign unique IDs.

The symbol cache in the NativeSession will also require the ability to clone
raw symbols, so I've provided implementations for that as well.

llvm-svn: 306042