[AST Matchers] Fix bug in 'optionally' matcher wherein all previous bindings...

[AST Matchers] Fix bug in 'optionally' matcher wherein all previous bindings are cleared when all inner matchers fail.

Summary: The implementation of 'optionally' doesn't preserve bindings when none of the submatchers succeed. This patch adds a regression test for that behavior and fixes it.

Reviewers: aaron.ballman, sbenza

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75365