Object: make the following changes into SymbolRef

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!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139683 91177308-0d34-0410-b5e6-96231b3b80d8