[lldb] Improve memory usage by freeing CTF types (NFC)

Improve memory usage by reducing the lifetime of CTF types. Once a CTF
type has been converted to a (complete) LLDB type, there's no need to
keep it in memory anymore. For most types, we can free them right after
creating the corresponding LLDB types. The only exception is record
types, which are only completed lazily.

Differential revision: https://reviews.llvm.org/D156606