Reimplement LinkFunctionProtos in terms of GetLinkageResult. This fixes

the second half of link-global-to-func.ll and causes some minor changes in
messages.

There are two TODOs here.  First, this causes a regression in 
2008-07-06-AliasWeakDest.ll, which is now failing (so I xfailed it).  Anton,
I would really appreciate it if you could take a look at this.  It should be
a matter of adding proper alias support to GetLinkageResult, and was probably
already a latent bug that would manifest with globals.

The second todo is to reimplement LinkAlias in the same pattern as 
function and global linking.  This should be pretty straight-forward for 
someone who knows aliases, but isn't a requirement for correctness.

llvm-svn: 53548