[TableGen] Correctly generate implicit anonymous prototype defs in multiclasses

[TableGen] Correctly generate implicit anonymous prototype defs in multiclasses

Even within a multiclass, we had been generating concrete implicit anonymous
defs when parsing values (generally in value lists). This behavior was
incorrect, and led to errors when multiclass parameters were used in the
parameter list of the implicit anonymous def.

If we had some multiclass:

multiclass mc<string n> {

 ... : SomeClass<SomeOtherClass<n> >

The capture of the multiclass parameter 'n' would not work correctly, and
depending on how the implicit SomeOtherClass was used, either TableGen would
ignore something it shouldn't, or would crash.

To fix this problem, when inside a multiclass, we generate prototype anonymous
defs for implicit anonymous defs (just as we do for explicit anonymous defs).
Within the multiclass, the current record prototype is populated with a node
that is essentially: !cast<SomeOtherClass>(!strconcat(NAME, anon_value_name)).
This is then resolved to the correct concrete anonymous def, in the usual way,
when NAME is resolved during multiclass instantiation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198348 91177308-0d34-0410-b5e6-96231b3b80d8