Preprocessor: support __BIGGEST_ALIGNMENT__ macro

For compatibility with GCC (and because it's generally helpful information
otherwise inaccessible to the preprocessor). This appears to be canonically the
alignment of max_align_t (e.g. on i386, __BIGGEST_ALIGNMENT__ is 4 even though
vector types will be given greater alignment).

Patch mostly by Mats Petersson

llvm-svn: 228367