[WIP]Add DllImport Charset UTF8

[luqunl]

The purpose is to enable Charset.UTF8 support for DllImport, which will help customer to write cross-platform PInvoke.

Partial Fix https://github.com/dotnet/coreclr/issues/1012

Remaining problems:

1. UTF8 Array isn't support yet.
2. Update roslyn to support Charset UTF8
3. There are couple shift/reduce, reduce/reduce conflict for ilasm yy file
4. More testing

Open problems:

1. What's the native type for default char marshalling with [DllImport(CharSet = CharSet.UTF8)]
   Currently use U2 instead.
2. Whether UnmanagedType.LPTSTR should support UTF8?
   Currently LPTSTR only supports ANSI/Unicode
3. Whether UnmanagedType.AsAny should support UTF8?
   Currently AsAny doesn't support UTF8.
4. Ongoing support for UTF16(LE/BE), UTF32?

[luqunl]

nltOle isn't used..

[jkotas]

Closing stale PR.