[ORC] Use finer-grained and session locking in MachOPlatform to avoid deadlock.

In MachOPlatform, obtaining the link-order for a JITDylib requires locking the
session, but also needs to be part of a larger atomic operation that collates
initializer symbols tracked by the platform. Trying to do this under a separate
platform mutex leads to potential locking order issues, e.g.

T1 locks session then tries to lock platform to register a new init symbol
meanwhile
T2 locks platform then tries to lock session to obtain link order.

Removing the platform lock and performing all these operations under the session
lock eliminates this possibility.

At the same time we also need to collate init pointers from the
MachOPlatform::InitScraperPlugin, and we don't need or want to lock the session
for that. The new InitSeqMutex has been added to guard these init pointers, and
the session mutex is never obtained while the InitSeqMutex is held.