SymbolVendor: Move locking into the Symbol Files

Summary:
The last bit of functionality in SymbolVendor passthrough functions is
the locking the module mutex. While it may be nice doing the locking in
a central place, we weren't really succesful in doing that right now,
because some SymbolFile function could still be called without going
through the SymbolVendor. This meant in SymbolFileDWARF (the only
battle-tested symbol file implementation) roughly a half of the
functions was taking additional locks and another half was asserting
that the lock is already held. By making the SymbolFile responsible for
locking, we can at least make the situation in SymbolFileDWARF more
consistent.

Reviewers: clayborg, JDevlieghere, jingham, jdoerfert

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D65329

llvm-svn: 367298