MCAsmParser: change representation of MCAsmMacroParameter

Rather than using std::pair, create a structure to represent the type.  This is
a preliminary refactoring to enable required parameter handling.  Additional
state is needed to indicate required parameters.  This has a minor side effect
of improving readability by providing more accurate names compared to first and
second.
MCAsmParser: support required parameters

This enhances the macro parser to parse and handle parameter qualifications,
which is needed to support required formal parameters in macro definitions.  A
required parameter may not be defaulted (though providing a default value is
accepted with a warning).  This improves GAS compatibility.

Partially addresses PR9248.
Implement DR577

DR18 previously forebode typedefs to be used as parameter types if they
were of type 'void'.  DR577 allows 'void' to be used as a function
parameter type regardless from where it came.