Handle StringImms passed to extern functions in c backend
Handle StringImms passed to extern functions in c backend

Had void *, which isn't a good match for const char *


Former-commit-id: 4d7ddd745ae85e7e5945b943ae948588f135780f
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide