Revert "rustboot: Emit an error instead of asserting in trans when a T is passed by value" due to test failures

This reverts commit ee901858.

On gcc 4.5.1 (fedora 14 i686) rust_test_runtime.cpp fails to compile
with the following error:

  rt/test/rust_test_runtime.cpp:57:61: error: passing NULL to non-pointer argument 3 of ‘void rust_task::start(uintptr_t, uintptr_t, uintptr_t, size_t)’

The explicit cast works around the error.

Emit a sensible error message if one or more of the parameters isn't enough context to instantiate a polymorphic function

rustboot: Emit an error message instead of asserting on a type-param mismatch in tag constructors in patterns

rustboot: Use a less hacky method to report tag names in error messages, which works for resolve errors as well

Prevent Not_found in the unfortunate case when we have to "friendly_stringify" a tag type we haven't yet recorded into cx.ctxt_all_tag_info

Pass outptr as in rustboot. Probably need to revisit this ABI. Also disgusting hack employed. Please remove.

rustboot: If the user tries to instantiate a non-parametric type with type parameters, report the location of the error