Modules: Don't warn upon missing headers while reading the module map.

Instead, mark the module as unavailable so that clang errors as soon as
someone tries to build this module.

This works towards the long-term goal of not stat'ing the header files at all
while reading the module map and instead read them only when the module is
being built (there is a corresponding FIXME in parseHeaderDecl()).  However, it
seems non-trivial to get there and this unblock us and moves us into the right
direction.

Also changed the implementation to reuse the same DiagnosticsEngine.