[lldb/API] Turn SBCompileUnit::GetIndexForLineEntry into FindLineEntryIndex (NFC)

This patch renames the `SBCompileUnit::GetIndexForLineEntry` api to be
an overload of `SBCompileUnit::FindLineEntryIndex`

Differential Revision: https://reviews.llvm.org/D125594



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