folly/futures/test/WhileDoTest.cpp: avoid shadowing warnings
folly/futures/test/WhileDoTest.cpp: avoid shadowing warnings

Summary: Fix warnings exposed by the upstream-proposed -Wshadow-compatible-local option.

Reviewed By: nisardan

Differential Revision: D4041779

fbshipit-source-id: 375e485a30f01a0610d881fe8b3237cc062aedef

floxard/folly,rklabs/folly,floxard/folly,floxard/folly,facebook/folly,Orvid/folly,rklabs/folly,facebook/folly,Orvid/folly,Orvid/folly,facebook/folly,charsyam/folly,rklabs/folly,floxard/folly,facebook/folly,rklabs/folly,Orvid/folly,charsyam/folly,rklabs/folly,charsyam/folly,floxard/folly,charsyam/folly,facebook/folly,Orvid/folly