Fix a couple PDB vtable bugs

[trumank]

I started digging into some vtable issues and there seem to be quite a few beyond just these trivial ones.

• First fix is making QualifiedName default to :: separator like docs say it should which fixes VTable struct being incorrectly defined without :: e.g. MultiDerivedVTable
• Second fix is in parsing vtable data symbol with explicit base class: e.g. MultiDerived::`vftable'{for `InterfaceB'} was getting incorrectly parsed as vftable'{for `InterfaceB::VTable

Test binaries: pdb-vtable.zip

Before:

After:

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