Refine our --wrap implementation.

Before this patch we would copy foo into real_foo and wrap_foo into
foo. The net result is that __wrap_foo shows up twice in the symbol
table.

With this patch we:

* save a copy of __real_foo before copying foo.
* drop one of the __wrap_foo from the symbol table.
* if __real_foo was not undefined, add a *new* symbol with that content to
  the symbol table.

The net result is that

Anything using foo now uses __wrap_foo
Anything using __real_foo now uses foo.
Anything using __wrap_foo still does.

And the symbol table has foo, __wrap_foo and __real_foo (if defined).

Which I think is the desired behavior.

llvm-svn: 315097