disable MSAN instrumentation in SparseByteSet::contains
disable MSAN instrumentation in SparseByteSet::contains

Summary: `SparseByteSet` uses uninitialized memory tricks. Disable MSAN instrumentation to avoid false positives.

Reviewed By: Gownta

Differential Revision: D39990729

fbshipit-source-id: ffcac32810d21ab04ec844d0b5eaef7881787fda

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly