[AsmParser]Emit an error if a macro has two (or more) parameters sharing the same name

Introducing a new error to macro parameters' parsing:
currently, llvm-mc won't complain if a macro have two (or more) named params with the same name.
this behavior is false, as there's no merit in having some params sharing a name.
now, instead of tolerate such a phenomena - emit an appropriate error.

Differential Revision: https://reviews.llvm.org/D31674

llvm-svn: 299815