[glean] fix missing nested member expressions
[glean] fix missing nested member expressions

Summary:
the glean indexer wasn't recursing on member expressions, leading to missing xrefs.

the changed tests look like this:
```
nested.a.b;
nested.a.b.c;
```

we now add xrefs on both `.a`'s and the second `.b`

Changelog: [internal]

Reviewed By: pieterv

Differential Revision: D35568320

fbshipit-source-id: e6885d63e08b376b10762c7f6b86f6f6e90c55fa

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