Enhance the IR type lowering code to be much smarter about recursively lowering

Enhance the IR type lowering code to be much smarter about recursively lowering
types.  Fore xample, we used to lower:

struct bar { int a; };
struct foo {
 void (*FP)(struct bar);
} G;

to:

%struct.foo = type { {}* }

since the function pointer would cause recursive translation of bar and
we didn't know if that would get us into trouble.  We are now smart enough
to know that it is fine, so we get this type instead:

%struct.foo = type { void (i32)* }

Codegen still needs to be prepared for uncooperative types at any place,
which is why I let the maximally uncooperative code sit around for awhile to
help shake out the bugs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@135244 91177308-0d34-0410-b5e6-96231b3b80d8