Ignore synthesised reads for jump-to-definition

We synthesise variables for things like tuple patterns. For example,
this Ruby code:

    a, b = ...

becomes:

    __synth__0 = ...
    a = __synth__0[0]
    b = __synth__0[1]

The `__synth__` variables should be ignored when calculating
jump-to-definition information, since they don't appear in the original
source code.

ql/src/queries/analysis/Definitions.ql

@@ -43,7 +43,9 @@ newtype DefLoc =
     exists(Ssa::WriteDefinition w |
       write = w.getWriteAccess() and
       read = w.getARead().getExpr() and
-      not read.getLocation() = write.getLocation()
+      not read.getLocation() = write.getLocation() and
+      not read.isSynthesized()
+    )
     )
   }