[Modules] Warning for module declarations lacking 'framework' qualifier

When a module declaration for a framework lacks the 'framework'
qualifier, the listed headers aren't found (because there's no
trigger for the special framework style path lookup) and the module
is silently not built. This leads to frameworks not being modularized
by accident, which is pretty bad.

Add a warning and suggest the user to add the 'framework' qualifier
when we can prove that it's the case.

rdar://problem/39193062

llvm-svn: 333718