SymbolVendor: Introduce Module::GetSymbolFile

Summary:
This is the next step in avoiding funneling all SymbolFile calls through
the SymbolVendor. Right now, it is just a convenience function, but it
allows us to update all calls to SymbolVendor functions to access the
SymbolFile directly. Once all call sites have been updated, we can
remove the GetSymbolVendor member function.

This patch just updates the calls to GetSymbolVendor, which were calling
it just so they could fetch the underlying symbol file. Other calls will
be done in follow-ups.

Reviewers: JDevlieghere, clayborg, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D65435

llvm-svn: 367664