Now that isPointerType can return a pointer type, avoid stripping off typedef

information in the common case.  On this invalid code:

typedef float float4 __attribute__((vector_size(16)));
typedef int int4 __attribute__((vector_size(16)));
void test(float4 a, int4 *result, int i) {
    result[i] = a;
}

we now generate:
  t.c:5:15: error: incompatible types assigning 'float4' to 'int4'
instead of:
  t.c:5:15: error: incompatible types assigning 'float4' to 'int  __attribute__((vector_size(16)))'

This implements test/Sema/typedef-retain.c

llvm-svn: 39892