SymbolVendor: Move Symtab construction into the SymbolFile

Summary:
Instead of having SymbolVendor coordinate Symtab construction between
Symbol and Object files, make the SymbolVendor function a passthrough,
and put all of the logic into the SymbolFile.

Reviewers: clayborg, JDevlieghere, jingham, espindola

Subscribers: emaste, mgorny, arichardson, MaskRay, lldb-commits

Differential Revision: https://reviews.llvm.org/D65208

llvm-svn: 367086