Reapply "[CodeGen] Fix assignments of inline layouts into the byref structure"

When using blocks, a byref structure is created to represent the
closure. The "byref.layout" field of this structure is an i8*. However,
some 'inline' layouts are represented as i64's, not i8*'s.

Prior to r246985 we cast the i64 'inline' layout to an i8* before
assigning it into the byref structure. This patch brings the cast back
and adds a regression test.

The original version of this patch was too invasive. This version only adds the
cast to BuildByrefLayout.

Differential Revision: http://reviews.llvm.org/D15674

rdar://23713871

llvm-svn: 256190