Introduce redecl_iterator, used for iterating over the redeclarations of a FunctionDecl or VarDecl.

It iterates over all the redeclarations, regardless of the starting point. For example:

1) int f();
2) int f();
3) int f();

if you have the (2) FunctionDecl and call redecls_begin/redecls_end to iterate, you'll get this sequence:
(2)
(1)
(3)

The motivation to introduce this was that, previously, if (3) was a function definition,
and you called getBody() at (2), it would not return it, since getBody() iterated over the previous declarations only,
so it would only check (2) and (1).

llvm-svn: 75604