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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@51224 91177308-0d34-0410-b5e6-96231b3b80d8