Merge PCHWriterDecl.cpp's isRequiredDecl and CodeGenModule::MayDeferGeneration into a new function,

DeclIsRequiredFunctionOrFileScopedVar.

This function is part of the public CodeGen interface since it's essentially a CodeGen predicate that is also
needed by the PCH mechanism to determine whether a decl needs to be deserialized during PCH loading for codegen purposes.
This fixes current (and avoids future) codegen-from-PCH bugs.

llvm-svn: 109546