Fully qualify the renamed symbol if the shortened name is ambiguous.

Summary:
For example, when renaming `a::b::x::foo` to `y::foo` below, replacing
`x::foo()` with `y::foo()` can cause ambiguity. In such cases, we simply fully
qualify the name with leading `::`.
```
namespace a {
namespace b {
namespace x { void foo() {} }
namespace y { void foo() {} }
}
}

namespace a {
namespace b {
void f() { x::foo(); }
}
}
```

Reviewers: ilya-biryukov, hokein

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50189

llvm-svn: 338832