Use LC_DYLD_EXPORTS_TRIE to locate the dyld trie structure if present

The pointer to the dyld trie data structure which lldb needs to parse to get
"trampoline kinds" on Darwin used to be a field in the LC_DYLD_INFO load command. A
new load command was added recently dedicated to this purpose: LC_DYLD_EXPORTS_TRIE.
The format of the trie did not change, however. So all we have to do is use the new
command if present. The commands are supposed to be mutually exclusive, so I added
an lldb_assert to warn if they are not.

Differential Revision: https://reviews.llvm.org/D107673