Fix -Wshadow warning in sorted_vector_types
Fix -Wshadow warning in sorted_vector_types

Summary: `last` was already passed in as a variable, so switch this so there isn't a local variable at all.

Reviewed By: mlogan, yfeldblum

Differential Revision: D4615206

fbshipit-source-id: dd8e3157e54dc2abe3dfd97837ce327371f4f7b2

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