During name lookup, use redecl_iterator to walk over the redeclaration

During name lookup, use redecl_iterator to walk over the redeclaration
chain to determine whether any declaration of the given entity is
visible, eliminating the redundant (and less efficient)
getPreviousDeclaration() implementation.

This tweak uncovered an omission in the handling of
RedeclarableTemplateDecl, where we weren't making sure to search for
additional redeclarations of a template in other module files. Things
would be cleaner if RedeclarableTemplateDecl actually used Redeclarable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@147687 91177308-0d34-0410-b5e6-96231b3b80d8