[MsDemangle] Read entire chain of target names in special tables (#155630)

When there's a deep inheritance hierarchy of multiple C++ classes (see
below), then the mangled name of a VFTable can include multiple key
nodes in the target name.
For example, in the following code, MSVC will generate mangled names for
the VFTables that have up to three key classes in the context.
<details><summary>Code</summary>

```cpp
class Base1 {
  virtual void a() {};
};
class Base2 {
  virtual void b() {}
};

class Ind1 : public Base1 {};
class Ind2 : public Base1 {};

class A : public Ind1, public Ind2 {};

class Ind3 : public A {};
class Ind4 : public A {};

class B : public Ind3, public Ind4 {};

class Ind5 : public B {};
class Ind6 : public B {};

class C : public Ind5, public Ind6 {};

int main() { auto i = new C; }
```
</details> 

This will include `??_7C@@6BInd1@@Ind4@@Ind5@@@` (and every other
combination). Microsoft's undname will demangle this to "const
C::\`vftable'{for \`Ind1's \`Ind4's \`Ind5'}". Previously, LLVM would
demangle this to "const C::\`vftable'{for \`Ind1'}".

With this PR, the output of LLVM's undname will be identical to
Microsoft's version. This changes `SpecialTableSymbolNode::TargetName`
to a node array which contains each key from the name. Unlike
namespaces, these keys are not in reverse order - they are in the same
order as in the mangled name.