'MacroExpansionDecl' and 'MacroExpansionExpr' have many common methods.
Introduce a common base class 'FreestandingMacroExpansion' that holds
'MacroExpansionInfo'.
Factor out common expansion logic to 'evaluateFreestandingMacro'
function that resembles 'evaluateAttachedMacro'.
Eliminate another circular reference through macro expansion mangling
by adjusting the starting declaration context to ensure that it is from
a suitable "outer" context.
Fixes rdar://108511666.
