[Sema] Introduce MaximumAlignment value, to be used instead of magical constants

There is llvm::Value::MaximumAlignment, which is numerically
equivalent to these constants, but we can't use it directly
because we can't include llvm IR headers in clang Sema.
So instead, copy-paste the constant, and fixup the places to use it.

This was initially reviewed in https://reviews.llvm.org/D72998