Rename CGT::VerifyFuncTypeComplete to isFuncTypeConvertible since

Rename CGT::VerifyFuncTypeComplete to isFuncTypeConvertible since
it is a predicate, not an action.  Change the return type to be a bool,
not the incomplete member.  Enhace it to detect the recursive compilation
case, allowing us to compile Eli's testcase on llvmdev:

struct T {
 struct T (*p)(void);
} t;

into:

%struct.T = type { {}* }

@t = common global %struct.T zeroinitializer, align 8

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@134853 91177308-0d34-0410-b5e6-96231b3b80d8