compiler/types: Make key in subroutine_name more effective

compiler/types: Make key in subroutine_name more effective

Use the string itself as a key for searching -- and the internal
allocated name as a key when storing.

Because record_key_hash doesn't consider the name field, which is
the only used field for a SUBROUTINE type, the hash key was always
the same for all types.  Using the name fixes this.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23277>