Make -Z gen-crate-map usable for I/O

In #10422, I didn't actually test to make sure that the '-Z gen-crate-map'
option was usable before I implemented it. The crate map was indeed generated
when '-Z gen-crate-map' was specified, but the I/O factory slot was empty
because of an extra check in trans about filling in that location.

This commit both fixes that location, and checks in a "fancy test" which does
lots of fun stuff. The test will use the rustc library to compile a rust crate,
and then compile a C program to link against that crate and run the C program.
To my knowledge this is the first test of its kind, so it's a little ad-hoc, but
it seems to get the job done. We could perhaps generalize running tests like
this, but for now I think it's fine to have this sort of functionality tucked
away in a test.
auto merge of #10437 : alexcrichton/rust/fix-gen-crate-map, r=cmr

In #10422, I didn't actually test to make sure that the '-Z gen-crate-map'
option was usable before I implemented it. The crate map was indeed generated
when '-Z gen-crate-map' was specified, but the I/O factory slot was empty
because of an extra check in trans about filling in that location.

This commit both fixes that location, and checks in a "fancy test" which does
lots of fun stuff. The test will use the rustc library to compile a rust crate,
and then compile a C program to link against that crate and run the C program.
To my knowledge this is the first test of its kind, so it's a little ad-hoc, but
it seems to get the job done. We could perhaps generalize running tests like
this, but for now I think it's fine to have this sort of functionality tucked
away in a test.