Rewrite addSymbolWrap and applySymbolWrap. NFCI.

r317396 changed the way how we handle the -defsym option. The option is
now handled using the infrastructure for the linker script.

We used to handle both -defsym and -wrap using the same set of functions
in the symbol table. Now, we don't need to do that.

This patch rewrites the functions so that they become more straightforward.
The new functions directly handle -wrap rather than abstract it.

llvm-svn: 317426