[lldb/API] Add SBCompileUnit::GetIndexForLineEntry method to SB API

This patch adds a new `GetIndexForLineEntry` method to the `SBCompileUnit`
class. As the name suggests, given an `SBLineEntry` object, this will
return the line entry index within a specific compile unit.

This method can take a `exact` boolean that will make sure that the
provided line entry matches perfectly another line entry in the compile unit.

rdar://47450887

Differention Revision: https://reviews.llvm.org/D125437



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