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.
llvm-svn: 197485