Implement C++11 [dcl.align]p6-p8, and C11 6.7.5/7. This had to be split out of
the normal attribute-merging path, because we can't merge alignment attributes
without knowing the complete set of alignment attributes which apply to a
particular declaration.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175861 91177308-0d34-0410-b5e6-96231b3b80d8