SwiftCC: Perform physical layout when computing coercion types

We need to take type alignment padding into account whe computing physical
layouts.

The layout must be compatible with the input layout, offsets are defined in
terms of offsets within a packed struct which are computed in terms of the alloc
size of a type.

Usingthe store size we would insert padding for the following type for example:

struct {

  int3 v;
  long long l;
} __attribute((packed))

On x86-64 int3 is padded to int4 alignment. The swiftcc type would be
<{ <3 x float>, [4 x i8], i64 }> which is not compatible with <{ <3 x float>,
i64 }>.

The latter has i64 at offset 16 and the former at offset 20.

rdar://32618125

llvm-svn: 305956