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.
Before this commit, replace-version-placeholder hardcoded the path
defining CURRENT_RUSTC_VERSION (to avoid replacing it). After a refactor
moved the file defining it without changing the hardcoded path, the tool
started replacing the constant itself with the version number.
To avoid this from happening in the future, this changes the definition
of the constant to avoid the tool from ever matching it.
