Factor out repeated parsing of a member-declarator when parsing a

Factor out repeated parsing of a member-declarator when parsing a
member-declaration. In the process, fix a couple of bugs that had crept in
where we would parse the first and subsequent member-declarators differently
(in particular, we didn't accept an asm-label on a member function definition
within a class, and we would accept virt-specifiers and attributes in the wrong
order on the first declarator but not on subsequent ones).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199957 91177308-0d34-0410-b5e6-96231b3b80d8