Add Clang conversion to windows-rdl

[kennykerr]

This update adds the beginnings of Clang-based header-to-rdl conversion. Using RDL for your APIs is prefereable, but if you happen to have an existing API defined in some set of header files, you can use the new Clang builder to convert the C/C++ definitions into RDL definitions. For example, here's how you might turn an API defined in some .h file into a .winmd file for generating bindings in any supported language:

// Convert api.h to api.rdl
windows_rdl::clang()
    .input("api.h")
    .input("Windows.winmd")
    .output("api.rdl")
    .namespace("Sample")
    .write()
    .unwrap();

// Convert api.rdl to api.winmd
windows_rdl::reader()
    .input("api.rdl")
    .input("Windows.winmd")
    .output("api.winmd")
    .write()
    .unwrap();

Just a thin slice is currently working. Now we need to flesh this out to bring parity with the RDL reader/writer and generally support most/all of the quirks in the Windows SDK headers. I'll also be removing the header2rdl experimental tooling soon.

The clang reader in the example above will convert a simple header like this:

enum Name {
    One = 1,
    Two = 2,
};

Into the following RDL:

#[win32]
mod Sample {
    #[repr(i32)]
    enum Name {
        One = 1,
        Two = 2,
    }
}

The reader will then convert the RDL into the following metadata:

.namespace Sample
{
	.class public auto ansi sealed Sample.Name
		extends [mscorlib]System.Enum
	{
		.field private specialname rtspecialname int32 value__
		.field public static literal valuetype [enum.winmd]Sample.Name One = int32(1)
		.field public static literal valuetype [enum.winmd]Sample.Name Two = int32(2)

	}
}

[youyuanwu]

@kennykerr Is scraping headers into different partitions/namespaces supported? Headers can have interdependencies, and the win32-metadata has a lot of knobs to tune which symbol should go to which partition. Sub included header can be configured to go into a sub namespace. Do u plan to support this?