C structs.

This comes up when we have a function that takes a struct and is defined in a
C++ file and used in a C file.

Before this commit, we will generate byval for C++ and will expand the struct
for C, thus causing difference at IR level. We will use bitcast of function type
at the callsite, which causes the inliner to not inline the function.

This commit changes how we handle small C like structs at IR level, but at
backend, we should generate the same argument passing before and after the
commit.

Note that the condition for expanding is still over conservative. We should be
able to expand type that is spelled with “class” and types that are not C-like.
But this commit fixes the inconsistent argument passing between C/C++.

Reviewed by John.

rdar://20121030


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@234033 91177308-0d34-0410-b5e6-96231b3b80d8