[lldb/API] Use a valid LineEntry object in SBCompileUnit::FindLineEntryIndex

This patch updates `SBCompileUnit::FindLineEntryIndex` to pass a valid
`LineEntry` pointer to `CompileUnit::FindLineEntry`.

This caused `LineTable::FindLineEntryIndexByFileIndexImpl` to return its
`best_match` initial value (UINT32_MAX).

rdar://78115426

Differential Revision: https://reviews.llvm.org/D102658



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>