Modules: Make missing headers in a module.map a warning not an error.

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

A better long-term strategy might be to not stat 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 would be a temporary
solution to unblock us.

Also changed the implementation to reuse the same DiagnosticsEngine as
otherwise warnings can't be enabled or disabled with command-line flags.