[dllimport] Check for dtor references in functions

Destructor references are not modelled explicitly in the AST. This adds
checks for destructor calls due to variable definitions and temporaries.

If a dllimport function references a non-dllimport destructor, it must
not be emitted available_externally, as the referenced destructor might
live across the DLL boundary and isn't exported.

llvm-svn: 295258