Convert CoreInputGraph.

Convert CoreInputGraph.

This is a part of InputGraph cleanup to represent input files as a flat
list of Files (and some meta-nodes for group etc.)

We cannot achieve that goal in one gigantic patch, so I split the task
into small steps as shown below.

(Recap the progress so far: Currently InputGraph contains a list of
InputElements. Each InputElement contain one File (that used to have
multiple Files, but I eliminated that use case in r223867). Files are
currently instantiated in Driver::link(), but I already made a change
to separate file parsing from object instantiation (r224102), so we
can safely instantiate Files when we need them, instead of wrapping
a file with the wrapper class (FileNode class). InputGraph used to
act like a generator class by interpreting groups by itself, but it's
now just a container of a list of InputElements (r223867).)

1. Instantiate Files in the driver and wrap them with WrapperNode.
WrapperNode is a temporary class that allows us to instantiate Files
in the driver while keep using the current InputGraph data structure.

This patch demonstrates how this step 1 looks like, using Core driver
as an example.

2. Do the same thing for the other drivers.

When step 2 is done, an InputGraph consists of GroupEnd objects or
WrapperNodes each of which contains one File. Other types of
FileNode subclasses are removed.

3. Replace InputGraph with std::vector<std::unique_ptr<InputElement>>.
InputGraph is already just a container of list of InputElements,
so this step removes that needless class.

4. Remove WrapperNode.

We need some code cleanup between each step, because many classes
do a bit odd things (e.g. InputGraph::getGroupSize()). I'll straight
things up as I need to.

llvm-svn: 225313