Move the code for loading metadata from rlibs and dylibs from
rustc_metadata into rustc_trans, and introduce a trait to avoid
introducing a direct dependency on rustc_trans.

This means rustc_metadata is no longer rebuilt when LLVM changes.

Consequently, session creation can no longer initialize LLVM.
The few places that use the compiler without going through
rustc_driver/CompilerCalls thus need to be careful to manually
initialize LLVM (via rustc_trans!) immediately after session
creation.

This means librustc is not rebuilt when LLVM changes.

This does not actually improve build times, since it still depends
on rustc_trans, but is better layering and fits the multi-backend
future slightly better.