entry: disallow explicit/interior mutability for read-only storage classes.

[eddyb]

Fixes #1008 by reinstating a mutability check, but with two key differences:

• immutable references are usable with all storage classes, not just read-only ones
  • _{not a difference from just before this PR, but rather pre-#567 the check was stricter (e.g. requiring &mut for StorageBuffer)}
  • (SPIR-V does not observe a distinction, anyway, it looks like a &mut that was never written to)
• interior mutability is taken into account, not just explicit &mut T
  • the &mut T-vs-&T distinction is still around, but only for syntax/phrasing in diagnostics
  • _{(interior mutability involves &T where T: !Freeze, with Freeze being a rustc-internal trait for "contains no UnsafeCells, at any inline nesting, ignoring behind pointer indirection", i.e. T: !Freeze implies T contains UnsafeCell, which would allow writing to those parts of &T}

Diagnostics example:
^{(the messages were surprisingly tricky to phrase - interior mutability means "expected &T"/"found &mut T" isn't the right distinction, as when T: !Freeze, they are both theoretically mutable)}