[Sema] Teach -Wcast-align to compute an accurate alignment using the

alignment information on VarDecls in more cases

This commit improves upon https://reviews.llvm.org/D21099. The code that
computes the source alignment now understands array subscript
expressions, binary operators, derived-to-base casts, and several more
expressions.

rdar://problem/59242343

Differential Revision: https://reviews.llvm.org/D78767