[CodeGen] Fix an assert in CodeGenFunction::EmitFunctionEpilog

The assert is triggered because isObjCRetainableType() is called on the
canonicalized return type that has been stripped of the typedefs and
attributes attached to it. To fix this assert, this commit gets the
original return type from CurCodeDecl or BlockInfo and uses it instead
of the canoicalized type.

rdar://problem/24470031

Differential Revision: http://reviews.llvm.org/D16914

llvm-svn: 261151