[pseudo] Eliminate an ambiguity for the empty member declaration.

We happened to introduce a `member-declaration := ;` rule
when inlining the `member-declaration := decl-specifier-seq_opt
member-declarator-list_opt ;`.
And with the `member-declaration := empty-declaration` rule, we had two parses of `;`.

This patch is to restrict the grammar to eliminate the
`member-declaration := ;` rule.

Differential Revision: https://reviews.llvm.org/D131724