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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201629 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201630 91177308-0d34-0410-b5e6-96231b3b80d8