Merge changes from SharpDevelop repository.
Add ConvertSymbol() to TypeSystemAstBuilder and IAmbience.
Swap actions in "The parameter '{0}' cannot be resolved" issue -- I think fixing an incorrect parameter name is the more common usecase.
CS0029InvalidConversionIssue: fields and local variables don't have a "return type", just a "type".
Remove implicit conversion from AstType to Expression.

Creating a TypeReferenceExpression can throw an exception if the AstType already is part of another tree, so it shouldn't be an implicit conversion.
For the common case of constructing a MemberReferenceExpression, use the AstType.Member() helper method instead.