[TrailingObjects] Dynamically realign under-aligned trailing objects.

[TrailingObjects] Dynamically realign under-aligned trailing objects.

Previously, the code enforced non-decreasing alignment of each trailing
type. However, it's easy enough to allow for realignment as needed, and
thus avoid the developer having to think about the possiblilities for
alignment requirements on all architectures.

(E.g. on Linux/x86, a struct with an int64 member is 4-byte aligned,
while on other 32-bit archs -- and even with other OSes on x86 -- it has
8-byte alignment. This sort of thing is irritating to have to manually
deal with.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256533 91177308-0d34-0410-b5e6-96231b3b80d8