Remove the declspecs from small alignments that we can force with

Remove the declspecs from small alignments that we can force with
a union. These don't actually work for by-value function arguments, and
MSVC warns if they exist even while (we hope) it aligns the argument
correctly due to the other union member.

This means MSVC will miss out on optimizations based on the alignment of
the buffer, but really, there aren't that many for x86 and MSVC is
likely not doing a great job of optimizing LLVM and Clang anyways.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171328 91177308-0d34-0410-b5e6-96231b3b80d8