[CodeGen][ObjC] Use the type of the captured field of the enclosing

block or lambda.

This is a follow-up to r281682, which fixed a bug in computeBlockInfo
where the captured VarDecl's type, rather than the captured field type
of the enclosing lambda or block, was used to compute the layout of a
block.

This commit makes similar changes to enterBlockScope. This is necessary
to correctly determine whether a block capture requires cleanup.

rdar://problem/30388124

llvm-svn: 295034