[clang] NFC: test for undefined behaviour in RawComment::getFormattedText()

This diff adds testcase for the issue fixed in https://reviews.llvm.org/D77468
but regression test was not added in the diff. On Clang 9 it caused
crash in cland during code completion.

Test Plan: check-clang-unit

Differential Revision: https://reviews.llvm.org/D103722