[MS Volatile] Don't make volatile loads/stores to underaligned objects atomic

Underaligned atomic LValues require libcalls which MSVC doesn't have.
MSVC doesn't seem to consider such operations as requiring a barrier
anyway.

This fixes PR27843.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@270576 91177308-0d34-0410-b5e6-96231b3b80d8