This is important for performance diagnostics: it’s assumed that (non-generic) MemoryLayout constants do not need to create metadata at runtime. At Onone this is only guaranteed if the TargetConstantFolding pass runs.
rdar://94836837
* move the source file to SILOptimizer/IRGenTransforms
* add a file level comment
* document and verify that the pass runs after serialization
* catch overflows when truncating a constant value
