SymbolVendor: Remove the object file member variable

Summary:
The last responsibility of the SymbolVendor was to hold an owning
reference to the object file (in case symbols are being read from a
different file than the main module). As SymbolFile classes already hold
a non-owning reference to the object file, we can easily remove this
responsibility of the SymbolVendor by making the SymbolFile reference
owning.

Reviewers: JDevlieghere, clayborg, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D65401

llvm-svn: 367392