[AST Matchers] Restrict `optionally` matcher to a single argument.

Summary:
Currently, `optionally` can take multiple arguments, which commits it to a
particular strategy for those arguments (in this case, "for each"). We limit the
matcher to a single argument, which avoids any potential confusion and
simplifies the implementation. The user can retrieve multiple-argument
optionality, by explicitly using the desired operator (like `forEach`, `anyOf`,
`allOf`, etc.) with all children wrapped in `optionally`.

Reviewers: sbenza, aaron.ballman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75556