[Clang] Propagate guaranteed alignment for malloc and others

LLVM should be smarter about *known* malloc's alignment and this knowledge may enable other optimizations.

Originally started as LLVM patch - https://reviews.llvm.org/D100862 but this logic should be really in Clang.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D100879