Fix support for _Bool bitfields. The issue is that the bitfield width

used for _Bool is not the same as the primitive width (which for _Bool 
is 1 bit).  The load and store changes add some casts to make the 
types consistent.  The EmitLValue changes make sure that the pointer is 
of an appropriate type for loading the bitfield.

This isn't perfect, but it's an improvement, and getting everything 
right depends on actually laying out structs in an ABI-compliant way.

llvm-svn: 51224