Add TooGeneric variant to LayoutError and emit Unknown one

- `check-pass` test for a MRE of #135020
- fail test for #135138
- switch to `TooGeneric` for checking CMSE fn signatures
- switch to `TooGeneric` for compute `SizeSkeleton` (for transmute)
- fix broken tests

compiler/rustc_transmute/src/layout/tree.rs

@@ -197,6 +197,7 @@ pub(crate) mod rustc {
             match err {
                 LayoutError::Unknown(..)
                 | LayoutError::ReferencesError(..)
+                | LayoutError::TooGeneric(..)
                 | LayoutError::NormalizationFailure(..) => Self::UnknownLayout,
                 LayoutError::SizeOverflow(..) => Self::SizeOverflow,
                 LayoutError::Cycle(err) => Self::TypeError(*err),