In Sema::MergeVarDecl we handle merging of storage classes and visibility

In Sema::MergeVarDecl we handle merging of storage classes and visibility
attributes. In cases where the merged declaration is fully equivalent to the
two original ones, some of the code was getLVForDecl was duplicated.

Cases that are still handled in getLVForDecl are things like

__private_extern__ int N;
int N;

For which we cannot produce a single merged decl with all the information.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167703 91177308-0d34-0410-b5e6-96231b3b80d8