Prevent compiler warning about mixing enumeral and non-enumeral types in ternary expression
Prevent compiler warning about mixing enumeral and non-enumeral types in ternary expression

Summary:
The GCC warning "warning: enumeral and non-enumeral type in conditional expression"
is logged a lot in many builds since lots of code depends on small_vector.  This diff
should prevent those warnings.

Reviewed By: yfeldblum

Differential Revision: D3762835

fbshipit-source-id: 49831e4364e716592287c05d1dbf1912326324f6

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