Add feature staging support to C++/WinRT#229
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Windows APIs typically need to be flighted such that teams working on different OS branches can use hold-back support to develop experimental or staged features while disabling those features from shipping builds (or different branches). This update adds support for OS developers to make use of feature staging metadata to elide component support for selected APIs.
There are a few points to consider:
Feature attributes only appear in internal metadata that is never shipped publicly. So in practice, this support is only for OS builds and there will never be any elision for components authored with published metadata.
Language projections always include all types in a given projection if those types appear in metadata, regardless of any Feature attributes that may be present.
The C++/WinRT component support for feature staging only holds back the implementation of feature-staged interfaces and runtime classes. The headers will still include types that are "always disabled", but a given component's generated scaffolding will not implement those types. This is intentional to avoid build instability and is consistent with how OS developers using WIL + WRL would manage this manually.