Don't verify inputs to the Linker if ODR merging.

This fixes pr28072.

The point, as Duncan pointed out, is that the file is already
partially linked by just reading it.

Long term I think the solution is to make metadata owned by the module
and then the linker will lazily read it and be in charge of all the
linking. Running a verifier in each input will defeat the lazy
loading, but will be legal.

Right now we are at the unfortunate position that to support odr
merging we cannot verify the inputs, which mildly annoying (see test
update).

llvm-svn: 274148