gccrs: Clear the substitution callbacks when copying ArgumentMappings

gccrs: Clear the substitution callbacks when copying ArgumentMappings

When we set the callback on substitutions this is not safe to be copied
around since we store the used argument mappings and can reuse them in
different contexts. This clears the callback on copy's to make it safer.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>
gcc/rust/ChangeLog:

	* typecheck/rust-tyty-subst.cc: update copy constructors