[clangd] Skip inline namespace when collecting scopes for index symbols.

Summary:
Some STL symbols are defined in inline namespaces. For example,
```
namespace std {
inline namespace __cxx11 {
 typedef ... string;
}
}
```
Currently, this will be `std::__cxx11::string`; however, `std::string` is desired.

Inline namespaces are treated as transparent scopes. This
reflects the way they're most commonly used for lookup. Ideally we'd
include them, but at query time it's hard to find all the inline
namespaces to query: the preamble doesn't have a dedicated list.

Reviewers: sammccall, hokein

Reviewed By: sammccall

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D42796

llvm-svn: 324065