[Orc] Add conversion to/from RuntimeDyld::SymbolInfo for JITSymbol.

This tidies up some code that was manually constructing RuntimeDyld::SymbolInfo
instances from JITSymbols. It will save more mess in the future when
JITSymbol::getAddress is extended to return an Expected<TargetAddress> rather
than just a TargetAddress, since we'll be able to embed the error checking in
the conversion.

llvm-svn: 271350