[NFC] Include SmallVector.h for Clang <= 5
[NFC] Include SmallVector.h for Clang <= 5

Clang 5 (possibly earlier) has a bug where it does not merge default
template arguments properly for forwarded declarations. Include
`SmallVector.h` directly in this case. This will slightly increase
compile times, but only for very old compilers and only for files that
don't already include `SmallVector.h` (of which there's currently only
23).

roambotics/swift,apple/swift,roambotics/swift,benlangmuir/swift,roambotics/swift,apple/swift,glessard/swift,glessard/swift,roambotics/swift,apple/swift,apple/swift,benlangmuir/swift,roambotics/swift,apple/swift,benlangmuir/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,glessard/swift,roambotics/swift,benlangmuir/swift,glessard/swift,glessard/swift,apple/swift