PR25910: clang allows two var definitions with the same mangled name

Proper diagnostic and resolution of mangled names' conflicts in variables.
When there is a declaration and a definition using the same name but different
types, we emit what is in the definition. When there are two conflicting
definitions, we issue an error.

Differential Revision: http://reviews.llvm.org/D15686

llvm-svn: 257754