libstdc++'s <cstdalign> #includes <stdalign.h> and expects it to guard against

libstdc++'s <cstdalign> #includes <stdalign.h> and expects it to guard against
being included in C++. Don't define alignof or alignas in this case. Note that
the C++11 standard is broken in various ways here (it refers to the contents
of <stdalign.h> in C99, where that header did not exist, and doesn't mention
the alignas macro at all), but we do our best to do what it intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175708 91177308-0d34-0410-b5e6-96231b3b80d8