[MergeFuncs] Fix callsite attributes in thunk generation

This change correctly sets the attributes on the callsites
generated in thunks. This makes sure things such as sret, sext, etc.
are correctly set, so that the call can be a proper tailcall.

Also, the transfer of attributes in the replaceDirectCallers function
appears to be unnecessary, but until this is confirmed it will remain.

Author: jrkoenig
Reviewers: dschuff, jfb
Subscribers: llvm-commits, nlewycky
Differential revision: http://reviews.llvm.org/D12581

llvm-svn: 247313