Most of the compiler should use SemanticAvailableAttr instead. In contexts like
ASTDumper where a semantic attribute is unavailable use accessors on
AvailableAttr.
NFC.
Put AvailabilityRange into its own header with very few dependencies so that it
can be included freely in other headers that need to use it as a complete type.
NFC.
Introduce a number of fixes to allow us to fully use declarations that
are produced by applying a peer macro to an imported declarations.
These changes include:
* Ensuring that we have the right set of imports in the source file
containing the macro expansion, because it depends only on the module
it comes from
* Ensuring that name lookup looks in that file even when the
DeclContext hierarchy doesn't contain the source file (because it's
based on the Clang module structure)
Expand testing to be sure that we're getting the right calls,
diagnostics, and generated IR symbols.
