[ManagedMemoryRewrite] Introduce a new pass to rewrite modules to use managed memory.

This pass is useful to automatically convert a codebase that uses malloc/free
to use their managed memory counterparts.

Currently, rewrite malloc and free to the `polly_{malloc,free}Managed` variants.

A future patch will teach ManagedMemoryRewrite to rewrite global arrays
as pointers to globally allocated managed memory.

Differential Revision: https://reviews.llvm.org/D36513

llvm-svn: 310471