This change adds a new IR noundef attribute, which denotes when a function call argument or return val may never contain uninitialized bits.
In MemorySanitizer, this attribute enables optimizations which decrease instrumented code size by up to 17% (measured with an instrumented build of clang) . I'll introduce the change allowing msan to take advantage of this information in a separate patch.
Tests as a separate patch:
https://reviews.llvm.org/D82317
This seems like a CodeGen-specific concern; I'm not sure this makes sense as a query on the Type.