[BuildLibCalls] Add noundef to allocator fns' size

This is a patch to explicitly mark the size parameter of allocator functions like malloc/realloc/... as noundef.

For C/C++: undef can be created from reading an uninitialized variable or padding.
Calling a function with uninitialized variable is already UB.
Calling malloc with padding value is.. something that's not expected. Padding bits may appear in a coerced aggregate, which doesn't apply to malloc's size.
Therefore, malloc's size can be marked as noundef.

For transformations that introduce malloc/realloc/..: I ran LLVM unit tests with an updated Alive2 semantics, and found no regression, so it seems okay.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D97045