wip patch: Create SymbolBody for all symbols

[Rafael Espíndola via llvm-commits]

> I expect that after this change, we will be removing many isLocal() function
> calls because in many places we will be able to handle local and global
> symbols in the same way (because they will be represented in the same data
> structure.) Does this affect the performance?

Maybe. I would expect it to still be impact by the fact that we always
look at member of a SymbolBody, instead of just comparing the value of
the pointer.

I will try to benchmark the followup patches.

Cheers,
Rafael