compiler: rename `variables` to `retainedNodes`

Part of the changes that was made to LLVM 6 by the time that LLVM 7 was released.
LLVM commit: 2c864551df
LLVM differential review: https://reviews.llvm.org/D45024

artiq/compiler/transforms/llvm_ir_generator.py

@@ -115,7 +115,7 @@ class DebugInfoEmitter:
             "scopeLine":       func.loc.line(),
             "isLocal":         func.is_internal,
             "isDefinition":    True,
-            "variables":   self.emit_metadata([])
+            "retainedNodes":   self.emit_metadata([])
         }, is_distinct=True)
 
     @memoize