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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102045 91177308-0d34-0410-b5e6-96231b3b80d8