Rollup merge of #110750 - scottmcm:vardebug-size, r=cjgillot

Add size asserts for MIR `SourceScopeData` & `VarDebugInfo`

There's vectors of both of these in `mir::Body`, so might as well track them.

(I was pondering adding something to one or the other, so wanted this to see the memory impact.)

Author: JohnTitor