[clangd] Change index scope convention from "outer::inner" to "outer::inner::"

Global scope is "" (was "")
Top-level namespace scope is "ns::" (was "ns")
Nested namespace scope is "ns::ns::" (was "ns::ns")

This composes more naturally:
 - qname = scope + name
 - full scope = resolved scope + unresolved scope (D42073 was the trigger)
It removes a wart from the old way: "foo::" has one more separator than "".

Another alternative that has these properties is "::ns", but that lacks
the property that both the scope and the name are substrings of the
qname as produced by clang.

llvm-svn: 322996