Fix interaction of max_align_t and modules.

When building with modules enabled, we were defining max_align_t as a typedef
for a different anonymous struct type each time it was included, resulting in
an error if <stddef.h> is not covered by a module map and is included more than
once in the same modules-enabled compilation of C11 or C++11 code.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218931 91177308-0d34-0410-b5e6-96231b3b80d8