Object: make the following changes into SymbolRef

- Add enum SymbolType and function getSymbolType()
- Add function isGlobal() - it's returns true for symbols that can be used in another objects, such as library functions.
- Rename function getAddress() to getOffset() and add new function getAddress(), because currently getAddress() returns section offset of symbol first byte. new getAddress() return symbol address.
- Change usage SymbolRef::getAddress() to getOffset() in tools/llvm-nm and tools/llvm-objdump.

Patch by Danil Malyshev!

llvm-svn: 139683