Fix crash in MergeRealloc on LLVM 4 (#1933)
Fix crash in MergeRealloc on LLVM 4 (#1933)

This crash occurred when the LLVM passes decided to optimise out the
declaration of a runtime function needed by the MergeRealloc pass.

This change fixes the bug by redeclaring the functions when
MergeRealloc needs them and can't find them.
Theodus/ponyc,CausalityLtd/ponyc,Theodus/ponyc,sgebbie/ponyc,Perelandric/ponyc,Perelandric/ponyc,dipinhora/ponyc,cquinn/ponyc,sgebbie/ponyc,dipinhora/ponyc,Praetonus/ponyc,Perelandric/ponyc,sgebbie/ponyc,boemmels/ponyc,jemc/ponyc,jemc/ponyc,Perelandric/ponyc,boemmels/ponyc,Praetonus/ponyc,kulibali/ponyc,ponylang/ponyc,boemmels/ponyc,jemc/ponyc,sgebbie/ponyc,dipinhora/ponyc,Praetonus/ponyc,Theodus/ponyc,kulibali/ponyc,kulibali/ponyc,sgebbie/ponyc,cquinn/ponyc,cquinn/ponyc,ponylang/ponyc,CausalityLtd/ponyc,Theodus/ponyc,Theodus/ponyc,CausalityLtd/ponyc,Praetonus/ponyc,cquinn/ponyc,ponylang/ponyc,Perelandric/ponyc,kulibali/ponyc,boemmels/ponyc,boemmels/ponyc