Keep track of which modules have been loaded directly (e.g., via

Keep track of which modules have been loaded directly (e.g., via
-import-module) vs. loaded because some other module depends on
them. As part of doing this, pass down the module that caused a module
to be loaded directly, rather than assuming that we're loading a
chain. Finally, write out all of the directly-loaded modules when
serializing an AST file (using the new IMPORTS record), so that an AST
file can depend on more than one other AST file, all of which will be
loaded when that AST file is loaded. This allows us to form and load a
tree of modules, but we can't yet load a DAG of modules.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137923 91177308-0d34-0410-b5e6-96231b3b80d8