Translate the metadata for the generic requirements of an extension context
into a demangle tree that is associated with the demangling of an extension.
Teach the ASTDemangler how to handle class layout constraints as well.
With this, RemoteAST can resolve types nested within most constrained
extensions.
When reading a mangled name, make sure to cope with embedded null bytes that
show up in symbolic references. When demangling such a name, handle symbolic
references.
Read the extended context mangled name from an extension context descriptor
so we can form a proper demangle tree for extensions. For example, this allows
types nested within extensions of types from different modules to be found.
