ELF: Implement --wrap.

In this patch, all symbols are resolved normally and then wrap options
are applied. Renaming is implemented by mutating `Body` pointers of
Symbols. (As a result, Symtab.find(SymbolName)->getName() may return
a string that's different from SymbolName, but that is by design.
I designed the symbol and the symbol table to allow this kind of
operations.)

http://reviews.llvm.org/D15896

llvm-svn: 257075