Dan Zheng 6c29939f59 [AutoDiff upstream] Add @differentiable ASTScope support. (#29171) ...

`@differentiable` attributes may contain `where` clauses referencing generic
parameters from some generic context, just like `@_specialize` attributes.

Without special ASTScope support for `@differentiable` attributes,
ASTScopeLookup.cpp logic tries to resolve the generic parameter `DeclName`s in
`where` clauses based on source location alone
(`ASTScopeImpl::findChildContaining`) and fails.

The fix is to add a special `DifferentiableAttributeScope`, mimicking
`SpecializeAttributeScope`. Every `@differentiable` attribute has its own scope,
derived from the declaration on which it is declared. Unlike `@_specialize`,
`@differentiable` may also be declared on `AbstractStorageDecl` declarations
(subscripts and variables).

Upstreams https://github.com/apple/swift/pull/27451.

Progress towards TF-828: upstream `@differentiable` attribute type-checking.

2020-01-13 17:48:44 -08:00

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