[clang-format] Ignore first token when finding MustBreak

When in ColumnLimit 0, the formatter looks for MustBreakBefore in the
line in order to check if a line is allowed to be merged onto one line.

However, since MustBreakBefore is really a property of the gap between
the token and the one previously, I belive the check is erroneous in
checking all the tokens in a line, since whether the previous line ended
with a forced line break should have no effect on whether the current
line is allowed to merge with the next one.

This patch changes the check to skip the first token in
`LineJoiner.containsMustBreak`.

This patch also changes a test, which is not ideal, but I believe the
test also suffered from this bug. The test case in question sets
AllowShortFunctionsOnASingleLine to "Empty", but the empty function in
said test case isn't merged to a single line, because of the very same
bug this patch fixes.

Fixes https://github.com/llvm/llvm-project/issues/62721

Reviewed By: HazardyKnusperkeks, owenpan, MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D150614