Make folly::getCurrentThreadId() return a thread ID on OSX
Make folly::getCurrentThreadId() return a thread ID on OSX

Summary: It was previously returning a pointer masquerading as a `uint64_t`.

Reviewed By: yfeldblum

Differential Revision: D4888325

fbshipit-source-id: 6cf6ed09f0e7a39a8a2f93d3fa14b06913c27805

facebook/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,Orvid/folly,Orvid/folly,Orvid/folly,Orvid/folly,Orvid/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly