Adds `#[rustc_force_inline]` which is similar to always inlining but
reports an error if the inlining was not possible, and which always
attempts to inline annotated items, regardless of optimisation levels.
It can only be applied to free functions to guarantee that the MIR
inliner will be able to resolve calls.
`rustc_codegen_llvm` and `rustc_codegen_gcc` duplicated logic for
checking if tied target features were partially enabled. This commit
consolidates these checks into `rustc_codegen_ssa` in the
`codegen_fn_attrs` query, which also is run pre-monomorphisation for
each function, which ensures that this check is run for unused functions,
as would be expected.
