[Clang][Modules] Merge availability attributes on imported decls

Currently we do not in general merge attributes when importing decls from modules. This patch handles availability, but long term we need to properly handle all attributes.

I tried to use Sema::mergeDeclAttributes, but it caused test crashes as I don't think it expects to be called in this context. We really shouldn't have duplicate code for merging attributes long term, but for now this fixes availability. There's already a TODO for this in the declaration of ASTDeclReader::mergeInheritableAttributes.

Differential Revision: https://reviews.llvm.org/D127182

rdar://85820301