* IInstr.cs (IInstr): Made this an abstract class and not an
	interface, added a .ctor which takes a Location.
	(IInstr.Location): New public readonly field.

	* CodeGen.cs (CodeGen.SymbolWriter): New public property.
	(CodeGen.BeginSourceFile, EndSourceFile): New public methods.
	(CodeGen.EndMethodDef): Take a `Location' argument.
	(CodeGen.Write): Create debugging info if the user requested it.

	* MethodDef.cs (MethodDef.ctor): Added `CodeGen codegen' and
	`Location start' arguments.  If the user requested debugging
	information, register ourselves with the symbol writer.
	(MethodDef.WriteCode): Emit debugging information.

	* DebuggingInfo.cs: New file.

svn path=/trunk/mcs/; revision=31488