[StackPromotion] fix the declaration of swift_buffer[De]AllocateOnStack.
[StackPromotion] fix the declaration of swift_buffer[De]AllocateOnStack.

This caused a linker failure on linux for some stdlib unit tests.
rdar://problem/23849414

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift