Start deciding earlier what to link.

Start deciding earlier what to link.

A traditional linker is roughly split in symbol resolution and "copying
stuff".

The two tasks are badly mixed in lib/Linker.

This starts splitting them apart.

With this patch there are no direct call to linkGlobalValueBody or
linkGlobalValueProto. Everything is linked via WapValue.

This also includes a few fixes:
* A GV goes undefined if the comdat is dropped (comdat11.ll).
* We error if an internal GV goes undefined (comdat13.ll).
* We don't link an unused comdat.

The first two match the behavior of an ELF linker. The second one is
equivalent to running globaldce on the input.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254336 91177308-0d34-0410-b5e6-96231b3b80d8