[lldb] Support recursive record types in CTF

Support recursive record types in CTF, for example a struct that
contains a pointer to itself:

  struct S {
    struct S *n;
  };

We are now more lazy when creating LLDB types. When encountering a
record type (struct or union) we create a forward declaration and only
complete it when requested.

Differential revision: https://reviews.llvm.org/D156498