[xbuild] Fix bug #663159 .

[xbuild] Fix bug #663159 .

Emit location information in error messages for condition parsing
errors.

	* Microsoft.Build.BuildEngine/Token.cs: Add position information.
	* Microsoft.Build.BuildEngine/ConditionParser.cs: Track api changes.
	Improve error messages.
	(ExpectToken): Rename to ..
	(IsAtToken): .. this, to better reflect its behavior.
	* Microsoft.Build.BuildEngine/ConditionTokenizer.cs: Track api
	changes and improve error messages.
	* Microsoft.Build.BuildEngine/ConditionFactorExpresion.cs: Track api
	changes