When inferring a submodule ID during module creation, look up the

include stack to find the first file that is known to be part of the
module. This copes with situations where the module map doesn't
completely specify all of the headers that are involved in the module,
which can come up when there are very strange #include_next chains
(e.g., with weird compiler/stdlib headers like stdarg.h or float.h).

llvm-svn: 147662