[clang-format] Fix assertion failure/crash with `AllowShortFunctionsOnASingleLine...

[clang-format] Fix assertion failure/crash with `AllowShortFunctionsOnASingleLine: Inline/InlineOnly`.

Fixes https://github.com/llvm/llvm-project/issues/54147.

When handling `AllowShortFunctionsOnASingleLine`, we were searching for the last line with a smaller level than the current line. The search was incorrect when the first line had the same level as the current one. This led to an unsatisfied assumption about the existence of a brace (non-comment token).

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D120902