Preprocessor: support __BIGGEST_ALIGNMENT__ macro

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228367 91177308-0d34-0410-b5e6-96231b3b80d8