Fix clippy's const fn stability check for CURRENT_RUSTC_VERSION

Fix clippy's const fn stability check for CURRENT_RUSTC_VERSION

Since clippy can use a projects MSRV for its lints, it might not want
to consider functions as const stable if they have been added lately.

Functions that have been stabilized this version use
CURRENT_RUSTC_VERSION as their version, which gets then turned into the
current version, which might be something like `1.66.0-dev`. The version
parser cannot deal with this version, so it has to be stripped off.