scip symbol = USR

Summary: High-level goal: use glass as a remote index store for sourcekit LSP. To that end, the next diff add thrift method usrToDefinition which takes a sourcekit USR (unique symbol identifier in clang/swift) and return the symbol definition. This means glean needs to store USR in the scip schema somewhere. Currently, we use as scip.Symbol a string like `scip . path/to/symbol/target . USR`. There was no strong motivation to use that format, it does not even comply with the SCIP specification for symbol IDs [here](https://github.com/sourcegraph/scip/blob/main/scip.proto#L147). So instead I'm using just a plain USR which should be unique by design.

Differential Revision: D74489678

fbshipit-source-id: 3857e7e2482d1722fccb20c8c0148a79c6d51baf

Author: Catherine Gasnier

glean/lang/swift/Glean/Indexer/Swift.hs

@@ -47,7 +47,8 @@ indexer = Indexer {
            [ "--output-dir", outDir
             , "--target", target
             , "--swift-only"
-            , "--output-type", "scip" ],
+            , "--output-type", "scip"
+            , "--build-indexer" ],
         scipRoot = indexerRoot,
         scipWritesLocal = False,
         scipLanguage = Just SCIP.Swift