Fix subtle bug in generating LLVM function declarations for builtin functions.

The decl wasn't being passed down, which meant that function attributes were not
being set correctly. This is particularly important for ARM, since it wants to
override the calling convention. Instead we would emit the builtin with the
wrong calling convention, and instcombine would come along and merrily shred all
the calls to it. :)

llvm-svn: 81756