IRgen: Rewrite bit-field access policy to not access data beyond the bounds of...

IRgen: Rewrite bit-field access policy to not access data beyond the bounds of the structure, which we also now verify as part of the post-layout consistency checks.
 - This fixes some pedantic bugs with packed structures, as well as major problems with -fno-bitfield-type-align.

 - Fixes PR5591, PR5567, and all known -fno-bitfield-type-align issues.

 - Review appreciated.

llvm-svn: 102045